I have a really weird one with HDR - HDR has worked fine on my PS5 and Series X and I am using the same cable on my PC as I did with those 2.
One thing I have noticed that I find strange - if I turn HDR on my PS5 the TV will flag in Game Bar or in the top right that a HDR signal is active/being received.
When I turn HDR on in Windows the screen flashes black as it turns on but my TV doesnt seem to recognise or acknowledge that there is a HDR signal there - I have the enhanced HDMI port on and plugged into the 2.1 port as well.
Auto-HDR has broken my machine too much for me to want to try it again - I did really like that feature on Xbox though so its a shame.
Another weird thing I have found is my GPU specs (Sapphire Pulse 9070XT) doesnt mention HDMI 2.1 at all - just says it has HDMI and Display Port 2.1 - would it be better for me to use a Display port to HDMI cable?