Make sure your system is fully up-to-date. There was a Testing update yesterday:
That update includes kwin version 6.5.4-3, as shown in my logs:
[2025-12-13T16:28:53+1100] [ALPM] upgraded kwin (6.5.4-1 -> 6.5.4-3)
That hopefully should fix the issue:
The last few comments from that bug report:
Comment 30 Belanthiel 2025-12-12 20:59:51 UTC
Installed the updated kwin 6.5.4-2.1 and it does allow the PC to wake from sleep now. It does not however allow the video to recover if I set KDE to dim and then turn off the monitor. I have to unplug the HDMI to regain video (6900XT) when this happens. Till this is resolved I have disabled both dim and turn off monitor in KDE settings.
Comment 31 Perfect Gentleman 2025-12-13 05:09:37 UTC
I’ve set “Turn off screen” to “Never” so far it’s okay.
Comment 32 Belanthiel 2025-12-13 07:09:23 UTC
With kwin 6.5.4-3.1 everything seems to be working well again. Will let it sleep tonight with dim, and turn screen off enabled.