Was losing my mind last night because HDR wasn't working properly on my Series X after the latest update. It would turn on, and then immediately flicker off - with every game I played.
Tried new cables, new inputs, basically whatever I could. Even re-joined the insider program to get that insider update (hoping it would fix the problem, since this only started with the last update). No help, still would flicker off and everything would be dull and washed out because the xbox would still think that HDR was on.
Turns out what had happened was that the color space settings on the xbox had changed from 10-bit to 8-bit, for whatever reason. And that option is buried in a different settings page (fidelity), which I had not even thought to look at. As soon as I switched back though, everything started working properly. So if anybody has any issues like I had, make sure to check out that video fidelity page.