Created attachment 128076 [details]
xrandr output off laptop screen off, external 4k Monitor on

SUMMARY
I've a 4k monitor connected to a laptop (laptop display turned off). xrandr output attached. My global x11 scale factor is 150 %.   Plasma is configured to not goto sleep when connected to power.

STEPS TO REPRODUCE
1. do noting until lockscreen is shown

OBSERVED RESULT
1/3 on the right and lower part of the 4k Monitor is black.  Other 2/3 shows the my background picture (kite)

EXPECTED RESULT
lock screen shows the kite background picture scaled 150% and therefore fills the complete screen size (as done when the Plasma session is active).

SOFTWARE/OS VERSIONS
Linux: KDE Neon User
Operating System: KDE neon 5.18
KDE Plasma Version: 5.18.4
KDE Frameworks Version: 5.69.0
Qt Version: 5.14.2
Kernel Version: 5.3.0-46-generic
OS Type: 64-bit
Processors: 8 × Intel® Core™ i7-8565U CPU @ 1.80GHz
Memory: 15.4 GiB of RAM

ADDITIONAL INFORMATION

When I turn of the monitor when I stop working (xrandr output does not change!) and turn it on later, the lock screen fills the complete external monitor size as it should be.