I retrieve this accepted change for F42: Changes/PlymouthUseSimpledrm - Fedora Project Wiki
But:
grep UseSimpledrm /usr/share/plymouth/plymouthd.defaults
UseSimpledrmNoLuks=1
UseSimpledrmNoLuks instead of UseSimpledrm
The changelog of plymouth (plymouth-24.004.60-22.fc43.x86_64) shows:
* Thu Jul 03 2025 Hans de Goede <hansg@kernel.org> - 24.004.60-19
- Don't use simpledrm together with LUKS (rhbz#2359283, rhbz#2339072)
2339072 – Plymouth shows black screen before FDE password prompt when setting plymouth.use-simpledrm on Nvidia Optimus system (FDE == Full Disk Encryption)
What has changed is that before the modeset would happen (and the screen would go black) before the disk-unlock screen shows and now the disk-unlock screen shows earlier (as intended) and then the modeset happens while it is showing causing the screen to go black.
In addition, I guess that this bug is not related to having a dual GPU system, but only to the “primary” GPU.
This bug was seen also on a intel/AMD dual GPU.
The system used in this topic is an intel/nvidia dual GPU, the “primary” (connected to the displays) being intel:
inxi:
Device-1: Intel TigerLake-LP GT2 [Iris Xe Graphics] vendor: Lenovo
driver: i915 v: kernel arch: Xe ports: active: HDMI-A-1,eDP-1
empty: DP-1, DP-2, DP-3, DP-4 bus-ID: 00:02.0 chip-ID: 8086:9a49
Device-2: NVIDIA TU117M [GeForce MX450] vendor: Lenovo driver: nvidia
v: 580.119.02 arch: Turing pcie: speed: 2.5 GT/s lanes: 4 bus-ID: 05:00.0
chip-ID: 10de:1f97
I think thus that if the plymouth version is at least 24.004.60-19 you will not have a black screen before the FDE password.
For other systems having an nvidia “primary” GPU, plymouth.use-simpledrm should be set, or add the nvidia modules to the initramfs.
In any case, suppressing rhgb wins 
Related topic: Making dracut run after akmods? - #16 by ksvist