Created attachment 137266 [details]
Shows the available screen resolutions when the bug occurs. Compare with the list provided by kscreen-doctor.

SUMMARY
Secondary display (Dell P2421DC) connected to the laptop (Dell Latitude 7410) via USB-C with max resolution of 2560x1440. It normally works, but sometimes when the screen is going to power saving mode (when not moving the mouse for a while) and waking up again (e.g. by moving the mouse) to the lock screen the screen resolution is changed to 1920x1200.

In the screen settings module the maximum listed resolutions are then 1920x1200 and 2048x1080 (see screenshot). Only when unplugging and re-plugging the screen, the correct resolution (2560x1440) appears again.

However, `kscreen-doctor -o` includes all resolutions:
Output: 69 DP-2 enabled connected  DisplayPort Modes: 101:1024x768@60 116:800x600@60 129:640x480@60 2913:2560x1440@60! 2914:1920x1200@60* 2915:2048x1080@60 2916:1920x1080@60 2917:1920x1080@50 2918:1920x1080@60 2919:1920x1080@60 2920:1920x1080@50 2921:1920x1080@60 2922:1600x1200@60 2923:1280x1024@75 2924:1152x864@75 2925:1280x720@60 2926:1280x720@50 2927:1280x720@60 2928:1024x768@75 2929:800x600@75 2930:720x576@50 2931:720x480@60 2932:720x480@60 2933:640x480@75 2934:640x480@60 2935:720x400@70 77:1680x1050@60 84:1280x1024@60 94:1280x800@60 Geometry: 0,0 1920x1200 Scale: 1 Rotation: 1 

also `xrandr` reports that the resolution is available:
DP-2 connected 1920x1200+0+0 (normal left inverted right x axis y axis) 527mm x 296mm
   2560x1440     59.95 +
   1920x1200     59.88* 
   2048x1080     60.00  
   1920x1080     60.00    50.00    59.94  
   1920x1080i    60.00    50.00    59.94  
   1600x1200     60.00  
   1680x1050     59.95  
   1280x1024     75.02    60.02  
   1280x800      59.81  
   1152x864      75.00  
   1280x720      60.00    50.00    59.94  
   1024x768      75.03    60.00  
   800x600       75.00    60.32  
   720x576       50.00  
   720x480       60.00    59.94  
   640x480       75.00    60.00    59.94  
   720x400       70.08
   


STEPS TO REPRODUCE
1. Connect external screen
2. Wait for system to go to lock screen and then to turn of the external display
3. Wake up (e.g. by moues movement), lock screen appears in wrong resolution

OBSERVED RESULT
After the secondary screen turns on again, the resolution is changed to 1920x1200. Previous resolution of 2560x1440 cannot be selected from the screen settings module, although xrandr and kscreen-doctor show that they are available (the resolution is not part of the drop-down list of available resolutions)


EXPECTED RESULT
The resolution should not change to 1920x1200 after power savings mode of secondary screen and resolution of 2560x1440 should always be selectable.


SOFTWARE/OS VERSIONS
Operating System: Arch Linux
KDE Plasma Version: 5.21.3
KDE Frameworks Version: 5.80.0
Qt Version: 5.15.2
Kernel Version: 5.11.10-arch1-1
OS Type: 64-bit
Graphics Platform: X11
Processors: 8 × Intel® Core™ i5-10310U CPU @ 1.70GHz
Memory: 15.3 GiB of RAM
Graphics Processor: Mesa Intel® UHD Graphics