If there is activity being done by the OS at this time you could potentially [Ctrl][ALT][F2] to get a terminal and view the output. Actually you could try from a terminal to shutdown with systemctl poweroff or gnome-session-quit --power-off and view what the output in the terminal is during shutdown to see if the OS is still doing something.
[Edit] : Also there is this … [Systemd] Issue shutdown timeout concerning systemd … and this … Shut down laptop power off takes minutes - #23 by nightromantic, which seems to have some troubleshooting ideas we can try.