SUMMARY
When an application creates a window that is initially full screen, it gets shown only on screen 0, regardless of where the geometry or screen setting was. If the window was not created/mapped full screen, but made full screen after shown, it is located in the correct screen.

STEPS TO REPRODUCE
1. Connect a second monitor and extend it (to the right for me)
2. Press Ctrl+Alt+Del to show the Leave window

This can also be done with mpv -screen=1 -fs=yes

OBSERVED RESULT
Both Leave windows are on screen 0 (to the left for me) and screen 1 is still usable. You can notice this because they're darker than usual and you can Alt+ or Meta+drag one of them to the other screen and you'll see another window below it.

EXPECTED RESULT
One Leave window is displayed in each screen. In the mpv case, it's displayed in the screen that was requested.

SOFTWARE/OS VERSIONS
OpenSUSE Tumbleweed 20211021
KDE Plasma Version: 5.23.0
KDE Frameworks Version: 5.87.0
Qt Version: 5.12.2 + kde patches

ADDITIONAL INFORMATION
This did not happen with kwin 5.22.5.

Using xprop, you can tell that one of the Leave windows has this property:
WM_NORMAL_HINTS(WM_SIZE_HINTS):
                user specified location: 3840, 0
                user specified size: 3840 by 2160
                program specified resize increment: 2 by 2
                program specified base size: -2 by -2
                window gravity: Static

And yet it was displayed at location 0,0.