SUMMARY
I am not if this is specific to the greeter or just a multi-monitor issue in general as explained in the additional information section.

After clicking shutdown, restart or logout in the system menu it should launch the logout greeter to confirm or cancel on ALL monitors connected to your system. This is no longer happening which due to my setup means I can not always see/interact with that menu.

[Monitor 1 (Output is a clone of Monitor 3)] [Monitor 2] [Monitor 3 (Primary display)]

The greeter is only appearing on Monitor 2. However alt+tab menu shows 3 instances of the greeter window despite only being shown on 1 display https://i.imgur.com/paHd477.png


STEPS TO REPRODUCE
1. Login (Using xorg session)
2. Try shutdown while using a mutli-monitor setup and observe logout greeter behaviour.

OBSERVED RESULT
Logout greeter asking you to confirm your choice does not appear on primary monitor.

EXPECTED RESULT
All monitors connected to system should display logout greeter.


SOFTWARE/OS VERSIONS
Solus
KDE Plasma Version: 5.23.0
KDE Frameworks Version: 5.87.0
Qt Version: 5.15.3

Nvidia driver: 470.63.01

ADDITIONAL INFORMATION

Elder Scrolls Online has started opening on Monitor 2 which has only started happening after upgrading to Plasma 5.23.0. I managed to solve this problem by changing ESO to use "Fullscreen" instead of "Fullscreen windowed". It still opens on monitor 2 now but will automatically move to my primary display after a couple of seconds with this option enabled. Unable to reproduce with other games so not sure if not opening on the correct display is just a coincidence.

After login the desktop wallpaper of the primary display uses the default wallpaper of my distribution for about 20 seconds before it applies my custom wallpaper. Monitor 2 always has my custom wallpaper set immediately upon login.