Are you using a DP -> HDMI converter? I know they can be temperamental.
Nope, those converters do not allow the HDMI 2.1 spec.
I’m running 6.10.x kernel on plasma 6.1.4. I’ve been bleeding edge because of the NVIDIA card for years now. I’ve been played with gamescope HDR, ICC profiles and everything you can think of since it was first possible.
I’ll have to play with it some more when I have time.