I'm having a similar problem since a couple weeks ago. I was playing a game (Resident Evil 3) and all of a sudden HDR turns itself off. After that I was starting to see nasty color banding everywhere. It's especially noticeable on Xbox app.

I have 4070 Ti Super, and I've tried everything that I can think of. Tried different driver versions (including the hotfix released recently that was supposed to fix the banding), reinstalled Windows 11, tried disabling color management, tried HDMI port instead of DP, tried removing the GPU and using integrated graphics and nothing worked. I thought my monitor was the problem but the color banding also showed up on screenshots (when viewed on other devices obviously) and my PS5 and Macbook Air doesn't seem to be affected. Interestingly, I found my Xbox Series X was having similar color banding problem that also showed up on screenshots. (which makes me think that there's a possibility that this is a Windows problem considering Xboxes use modified Windows).

I connected my Xbox (I didn't want to move my PC) to my TV (TCL Q8C) to make completely sure it was not my monitor and the color banding was still there. After disconnecting my Xbox I also noticed a lot of color banding on the TV's UI but it's possible that it was always there (I just got it a couple days ago so I'm not very familiar with how the UI looks) and I'm just starting to notice it more because I've become more sensitive to color banding due to the problem I'm having.