F39, Nvidia, Plasma: screens won't stay powered off

This system has one Nvidia GTX 1050 Ti card with two monitors — one connected via HDMI, one via DisplayPort. I run KDE Plasma. It’s configured to power off the monitors after five minutes of inactivity. However, as soon as the monitors have gone off, they come straight back on. The same happens if I turn them off manually. Suppose, at 19:10:45, I run this command:

sleep 1; xset dpms force off

I see this in the log:

May 31 19:10:46 localhost.localdomain kdeconnectd[4389]: org.kde.pulseaudio: No object for name "alsa_output.pci-0000_41_00.1.hdmi-stereo"
May 31 19:10:46 localhost.localdomain kdeconnectd[4389]: org.kde.pulseaudio: No object for name "alsa_output.pci-0000_0c_00.3.iec958-stereo.monitor"
May 31 19:10:46 localhost.localdomain kdeconnectd[4389]: org.kde.pulseaudio: No object for name "alsa_output.pci-0000_0c_00.3.iec958-stereo.monitor"
May 31 19:10:46 localhost.localdomain plasmashell[3982]: org.kde.plasma.pulseaudio: No object for name "alsa_output.pci-0000_41_00.1.hdmi-stereo"
May 31 19:10:46 localhost.localdomain plasmashell[3982]: org.kde.plasma.pulseaudio: No object for name "alsa_output.pci-0000_0c_00.3.iec958-stereo.monitor"
May 31 19:10:48 localhost.localdomain plasmashell[3982]: org.kde.plasma.pulseaudio: No object for name "alsa_output.pci-0000_0c_00.3.iec958-stereo.monitor"
May 31 19:11:05 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): EDID vendor "AOC", prod id 12921
May 31 19:11:05 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Using hsync ranges from config file
May 31 19:11:05 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Using vrefresh ranges from config file
May 31 19:11:05 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Printing DDC gathered Modelines:
May 31 19:11:05 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "2560x1440"x0.0  241.50  2560 2608 2640 2720  1440 1443 1448 1481 +hsync +vsync (88.8 kHz eP)
[...]
May 31 19:11:05 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "1280x960"x0.0  108.00  1280 1376 1488 1800  960 961 964 1000 +hsync +vsync (60.0 kHz e)
May 31 19:11:05 localhost.localdomain kwin_x11[3953]: kwin_core: Parse error in tiles configuration for monitor "5b7a5ba4-43cc-5910-8e98-f358f9d54b43" : "illegal value" Creating default setup
May 31 19:11:05 localhost.localdomain kded5[3952]: colord: Failed to register device: "device id 'xrandr-Q3279WG5B-61983' already exists"
May 31 19:11:05 localhost.localdomain kded5[3952]: colord: Setting X atom (id: 0 ) "_ICC_PROFILE" on output: "DP-1"
May 31 19:11:05 localhost.localdomain kded5[3952]: colord: EDID ICC Profile already exists "/home/msl/.local/share/icc/edid-62b58bd533759a8afaa820a861c846ec.icc"
May 31 19:11:06 localhost.localdomain plasmashell[3982]: file:///usr/share/plasma/wallpapers/org.kde.image/contents/ui/main.qml:20: TypeError: Cannot read property 'pluginName' of null
[...]
May 31 19:11:06 localhost.localdomain plasmashell[3982]: file:///usr/share/plasma/wallpapers/org.kde.image/contents/ui/main.qml:18: TypeError: Cannot read property 'configuration' of null
May 31 19:11:06 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (--) modeset(0): HDMI max TMDS frequency 300000KHz
May 31 19:11:06 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): EDID vendor "AOC", prod id 12921
May 31 19:11:06 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Using hsync ranges from config file
May 31 19:11:06 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Using vrefresh ranges from config file
May 31 19:11:06 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Printing DDC gathered Modelines:
May 31 19:11:06 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "2560x1440"x0.0  241.50  2560 2608 2640 2720  1440 1443 1448 1481 +hsync +vsync (88.8 kHz eP)
[...]
May 31 19:11:06 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "1280x960"x0.0  108.00  1280 1376 1488 1800  960 961 964 1000 +hsync +vsync (60.0 kHz e)
May 31 19:11:06 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Allocate new frame buffer 2560x1440 stride
May 31 19:11:06 localhost.localdomain kdeconnectd[4389]: org.kde.pulseaudio: No object for name "alsa_output.pci-0000_0c_00.3.iec958-stereo.monitor"
May 31 19:11:06 localhost.localdomain kdeconnectd[4389]: org.kde.pulseaudio: No object for name "alsa_output.pci-0000_0c_00.3.iec958-stereo.monitor"
May 31 19:11:06 localhost.localdomain plasmashell[3982]: file:///usr/share/plasma/look-and-feel/org.kde.breeze.desktop/contents/osd/Osd.qml:24:15: QML OsdItem (parent or ancestor of QQuickLayoutAttached): Binding loop detected for property "preferredWidth"
May 31 19:11:06 localhost.localdomain plasmashell[3982]: org.kde.plasma.pulseaudio: No object for name "alsa_output.pci-0000_0c_00.3.iec958-stereo.monitor"
May 31 19:11:06 localhost.localdomain plasmashell[3982]: org.kde.plasma.pulseaudio: No object for name "alsa_output.pci-0000_0c_00.3.iec958-stereo.monitor"
May 31 19:11:06 localhost.localdomain kded5[3952]: colord: Failed to register device: "device id 'xrandr-Q3279WG5B-61983' already exists"
May 31 19:11:06 localhost.localdomain kded5[3952]: colord: Setting X atom (id: 0 ) "_ICC_PROFILE" on output: "DP-1"
May 31 19:11:06 localhost.localdomain kded5[3952]: colord: Failed to register device: "device id 'xrandr-Q3279WG5B-61983' already exists"
May 31 19:11:06 localhost.localdomain kded5[3952]: colord: Setting X atom (id: 0 ) "_ICC_PROFILE" on output: "DP-1"
May 31 19:11:06 localhost.localdomain kded5[3952]: colord: Failed to register device: "device id 'xrandr-Q3279WG5B-61983' already exists"
May 31 19:11:06 localhost.localdomain kded5[3952]: colord: Setting X atom (id: 0 ) "_ICC_PROFILE" on output: "DP-1"
May 31 19:11:06 localhost.localdomain kwin_x11[3953]: kwin_core: XCB error: 152 (BadDamage), sequence: 64448, resource id: 14755219, major code: 143 (DAMAGE), minor code: 3 (Subtract)
May 31 19:11:06 localhost.localdomain kded5[3952]: colord: Failed to register device: "device id 'xrandr-Q3279WG5B-61983' already exists"
May 31 19:11:06 localhost.localdomain kded5[3952]: colord: Setting X atom (id: 0 ) "_ICC_PROFILE" on output: "DP-1"
May 31 19:11:06 localhost.localdomain kded5[3952]: colord: Failed to register device: "device id 'xrandr-Q3279WG5B-61983' already exists"
May 31 19:11:06 localhost.localdomain kded5[3952]: colord: Setting X atom (id: 0 ) "_ICC_PROFILE" on output: "DP-1"
May 31 19:11:06 localhost.localdomain kded5[3952]: colord: EDID ICC Profile already exists "/home/msl/.local/share/icc/edid-62b58bd533759a8afaa820a861c846ec.icc"
[...]
May 31 19:11:06 localhost.localdomain kded5[3952]: colord: EDID ICC Profile already exists "/home/msl/.local/share/icc/edid-62b58bd533759a8afaa820a861c846ec.icc"
May 31 19:11:07 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Allocate new frame buffer 5120x1440 stride
May 31 19:11:07 localhost.localdomain plasmashell[3982]: trying to show an empty dialog
May 31 19:11:07 localhost.localdomain kded5[3952]: colord: Setting X atom (id: 0 ) "_ICC_PROFILE" on output: "HDMI-1"
May 31 19:11:07 localhost.localdomain kded5[3952]: colord: Failed to register device: "device id 'xrandr-Q3279WG5B-61983' already exists"
May 31 19:11:07 localhost.localdomain kded5[3952]: colord: Setting X atom (id: 1 ) "_ICC_PROFILE_1" on output: "DP-1"
May 31 19:11:07 localhost.localdomain kded5[3952]: colord: Failed to register device: "device id 'xrandr-Q3279WG5B-61982' already exists"
[...]
May 31 19:11:08 localhost.localdomain kded5[3952]: colord: Failed to register device: "device id 'xrandr-Q3279WG5B-61983' already exists"
May 31 19:11:08 localhost.localdomain kded5[3952]: colord: Setting X atom (id: 1 ) "_ICC_PROFILE_1" on output: "DP-1"
May 31 19:11:08 localhost.localdomain kded5[3952]: colord: Failed to register device: "device id 'xrandr-Q3279WG5B-61982' already exists"
May 31 19:11:08 localhost.localdomain kded5[3952]: colord: Setting X atom (id: 0 ) "_ICC_PROFILE" on output: "HDMI-1"
May 31 19:11:08 localhost.localdomain plasmashell[3982]: file:///usr/share/plasma/plasmoids/org.kde.plasma.notifications/contents/ui/FullRepresentation.qml:183:13: QML DescriptiveLabel: Binding loop detected for property "verticalAlignment"
May 31 19:11:08 localhost.localdomain kded5[3952]: colord: Failed to register device: "device id 'xrandr-Q3279WG5B-61983' already exists"
May 31 19:11:08 localhost.localdomain kded5[3952]: colord: Setting X atom (id: 1 ) "_ICC_PROFILE_1" on output: "DP-1"
May 31 19:11:08 localhost.localdomain kded5[3952]: colord: EDID ICC Profile already exists "/home/msl/.local/share/icc/edid-5970f27cb7911d051066637cee86f4bb.icc"
[...]
May 31 19:11:08 localhost.localdomain kded5[3952]: colord: EDID ICC Profile already exists "/home/msl/.local/share/icc/edid-62b58bd533759a8afaa820a861c846ec.icc"
May 31 19:11:08 localhost.localdomain plasmashell[3982]: file:///usr/share/plasma/look-and-feel/org.kde.breeze.desktop/contents/osd/Osd.qml:24:15: QML OsdItem (parent or ancestor of QQuickLayoutAttached): Binding loop detected for property "preferredWidth"
May 31 19:11:10 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (--) modeset(0): HDMI max TMDS frequency 300000KHz
May 31 19:11:10 localhost.localdomain kwin_x11[3953]: kwin_core: Parse error in tiles configuration for monitor "5b7a5ba4-43cc-5910-8e98-f358f9d54b43" : "illegal value" Creating default setup
May 31 19:11:10 localhost.localdomain kded5[3952]: colord: Failed to register device: "device id 'xrandr-Q3279WG5B-61982' already exists"
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): EDID vendor "AOC", prod id 12921
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Using hsync ranges from config file
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Using vrefresh ranges from config file
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Printing DDC gathered Modelines:
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "2560x1440"x0.0  241.50  2560 2608 2640 2720  1440 1443 1448 1481 +hsync +vsync (88.8 kHz eP)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "2560x1440"x0.0  296.00  2560 2568 2600 2666  1440 1443 1448 1481 +hsync -vsync (111.0 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "1920x1080"x0.0  148.50  1920 2008 2052 2200  1080 1084 1089 1125 +hsync +vsync (67.5 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "720x480"x0.0   27.00  720 736 798 858  480 489 495 525 -hsync -vsync (31.5 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "720x576"x0.0   27.00  720 732 796 864  576 581 586 625 -hsync -vsync (31.2 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "1280x1440"x0.0  156.00  1280 1376 1512 1744  1440 1443 1453 1493 -hsync +vsync (89.4 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "1920x1080"x0.0  148.50  1920 2448 2492 2640  1080 1084 1089 1125 +hsync +vsync (56.2 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "1920x1080i"x0.0   74.25  1920 2008 2052 2200  1080 1084 1094 1125 interlace +hsync +vsync (33.8 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "1920x1080i"x0.0   74.25  1920 2448 2492 2640  1080 1084 1094 1125 interlace +hsync +vsync (28.1 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "1280x720"x0.0   74.25  1280 1390 1430 1650  720 725 730 750 +hsync +vsync (45.0 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "1280x720"x0.0   74.25  1280 1720 1760 1980  720 725 730 750 +hsync +vsync (37.5 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "640x480"x0.0   25.18  640 656 752 800  480 490 492 525 -hsync -vsync (31.5 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "800x600"x0.0   40.00  800 840 968 1056  600 601 605 628 +hsync +vsync (37.9 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "800x600"x0.0   36.00  800 824 896 1024  600 601 603 625 +hsync +vsync (35.2 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "640x480"x0.0   31.50  640 656 720 840  480 481 484 500 -hsync -vsync (37.5 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "640x480"x0.0   31.50  640 664 704 832  480 489 492 520 -hsync -vsync (37.9 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "640x480"x0.0   30.24  640 704 768 864  480 483 486 525 -hsync -vsync (35.0 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "720x400"x0.0   28.32  720 738 846 900  400 412 414 449 -hsync +vsync (31.5 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "1280x1024"x0.0  135.00  1280 1296 1440 1688  1024 1025 1028 1066 +hsync +vsync (80.0 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "1024x768"x0.0   78.75  1024 1040 1136 1312  768 769 772 800 +hsync +vsync (60.0 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "1024x768"x0.0   75.00  1024 1048 1184 1328  768 771 777 806 -hsync -vsync (56.5 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "1024x768"x0.0   65.00  1024 1048 1184 1344  768 771 777 806 -hsync -vsync (48.4 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "832x624"x0.0   57.28  832 864 928 1152  624 625 628 667 -hsync -vsync (49.7 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "800x600"x0.0   49.50  800 816 896 1056  600 601 604 625 +hsync +vsync (46.9 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "800x600"x0.0   50.00  800 856 976 1040  600 637 643 666 +hsync +vsync (48.1 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "1680x1050"x0.0  119.00  1680 1728 1760 1840  1050 1053 1059 1080 +hsync -vsync (64.7 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "1440x900"x0.0   88.75  1440 1488 1520 1600  900 903 909 926 +hsync -vsync (55.5 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "1280x1024"x0.0  108.00  1280 1328 1440 1688  1024 1025 1028 1066 +hsync +vsync (64.0 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (II) modeset(0): Modeline "1280x960"x0.0  108.00  1280 1376 1488 1800  960 961 964 1000 +hsync +vsync (60.0 kHz e)
May 31 19:11:11 localhost.localdomain /usr/libexec/gdm-x-session[3411]: (--) modeset(0): HDMI max TMDS frequency 300000KHz
May 31 19:11:11 localhost.localdomain kded5[3952]: colord: Setting X atom (id: 0 ) "_ICC_PROFILE" on output: "HDMI-1"
May 31 19:11:11 localhost.localdomain kded5[3952]: colord: Failed to register device: "device id 'xrandr-Q3279WG5B-61983' already exists"
May 31 19:11:11 localhost.localdomain kded5[3952]: colord: Setting X atom (id: 1 ) "_ICC_PROFILE_1" on output: "DP-1"
May 31 19:11:11 localhost.localdomain kded5[3952]: colord: Failed to register device: "device id 'xrandr-Q3279WG5B-61982' already exists"
May 31 19:11:11 localhost.localdomain kded5[3952]: colord: Setting X atom (id: 0 ) "_ICC_PROFILE" on output: "HDMI-1"
May 31 19:11:11 localhost.localdomain plasmashell[3982]: file:///usr/share/plasma/wallpapers/org.kde.image/contents/ui/main.qml:20: TypeError: Cannot read property 'pluginName' of null
May 31 19:11:11 localhost.localdomain plasmashell[3982]: file:///usr/share/plasma/wallpapers/org.kde.image/contents/ui/main.qml:75: TypeError: Cannot read property 'configuration' of null
May 31 19:11:11 localhost.localdomain plasmashell[3982]: file:///usr/share/plasma/wallpapers/org.kde.image/contents/ui/main.qml:78: TypeError: Cannot read property 'pluginName' of null
May 31 19:11:11 localhost.localdomain plasmashell[3982]: file:///usr/share/plasma/wallpapers/org.kde.image/contents/ui/main.qml:80: TypeError: Cannot read property 'configuration' of null
May 31 19:11:11 localhost.localdomain plasmashell[3982]: file:///usr/share/plasma/wallpapers/org.kde.image/contents/ui/main.qml:81: TypeError: Cannot read property 'configuration' of null
May 31 19:11:11 localhost.localdomain plasmashell[3982]: file:///usr/share/plasma/wallpapers/org.kde.image/contents/ui/main.qml:82: TypeError: Cannot read property 'configuration' of null
May 31 19:11:11 localhost.localdomain plasmashell[3982]: file:///usr/share/plasma/wallpapers/org.kde.image/contents/ui/main.qml:83: TypeError: Cannot read property 'configuration' of null
May 31 19:11:11 localhost.localdomain plasmashell[3982]: file:///usr/share/plasma/wallpapers/org.kde.image/contents/ui/main.qml:84: TypeError: Cannot read property 'configuration' of null
May 31 19:11:11 localhost.localdomain plasmashell[3982]: file:///usr/share/plasma/wallpapers/org.kde.image/contents/ui/main.qml:16: TypeError: Cannot read property 'configuration' of null
May 31 19:11:11 localhost.localdomain plasmashell[3982]: file:///usr/share/plasma/wallpapers/org.kde.image/contents/ui/main.qml:17: TypeError: Cannot read property 'configuration' of null
May 31 19:11:11 localhost.localdomain plasmashell[3982]: file:///usr/share/plasma/wallpapers/org.kde.image/contents/ui/main.qml:18: TypeError: Cannot read property 'configuration' of null
May 31 19:11:11 localhost.localdomain kded5[3952]: colord: Failed to register device: "device id 'xrandr-Q3279WG5B-61983' already exists"
May 31 19:11:11 localhost.localdomain kded5[3952]: colord: Setting X atom (id: 1 ) "_ICC_PROFILE_1" on output: "DP-1"
May 31 19:11:11 localhost.localdomain kded5[3952]: colord: Failed to register device: "device id 'xrandr-Q3279WG5B-61982' already exists"
May 31 19:11:11 localhost.localdomain kded5[3952]: colord: Setting X atom (id: 0 ) "_ICC_PROFILE" on output: "HDMI-1"
May 31 19:11:11 localhost.localdomain kded5[3952]: colord: Failed to register device: "device id 'xrandr-Q3279WG5B-61983' already exists"
May 31 19:11:11 localhost.localdomain kded5[3952]: colord: Setting X atom (id: 1 ) "_ICC_PROFILE_1" on output: "DP-1"
May 31 19:11:11 localhost.localdomain kded5[3952]: colord: EDID ICC Profile already exists "/home/msl/.local/share/icc/edid-5970f27cb7911d051066637cee86f4bb.icc"

(I’ve snipped some repeated lines in order to limbo under the limit of 32,000 characters.)

The problem started when I upgraded from D38 to F39 last weekend. I didn’t change the graphics card, the cables or the monitors. The other inputs of the monitors are connected to a work laptop running another popular OS, and that doesn’t have this problem.

How can I get power-saving to work the way it did in F38?

Thanks ever so much for replying. The problem turned out to be that power settings had got garbled during the upgrade from F38 to F39. I went into Plasma’s systemsetting app and, from there, went into “Power Management/Energy Saving”. In that screen, I turned off “Dim screen” (which had never been turned on before, and which I didn’t want) and turned on “Switch off after” (and set it to five minutes).

That was enough to get the monitors to turn off after five minutes, and it also prevents the monitors from coming on after a few seconds when I run

sleep 1; xset dpms force off

Thanks again for your help.