Are you using a DP -> HDMI converter? I know they can be temperamental.
You can try switching VTs back and forth (CTRL-ALT-F1 <-> CTRL-ALT-F7), and maybe switching resolution back and forth could help.
Make sure you have the latest kernel and try the latest Plasma 6. If all of that fails, you can try Steam with gamescope running in KMS mode (easiest way to test this would probably be Bazzite or Nobara if your distribution doesn’t support it), but it’s also a bit temperamental with enabling HDR.