It happens on 1440p120 too. I think PC users should avoid RGB and force YCbCr444.
I tried the following setups in PC mode:
RGB Full with TV set to HDMI Black Full = Banding/Posturization
RGB Limited with TV set to HDMI Black Low = Banding/Posturization
YCbCr444 with TV set to HDMI Black Low = No Banding/Posturization and 4:4:4 is kept
This matches what others have reported with HDR games and PC Mode. HDR just makes it more noticeable but it's definitely happening in even 8bit SDR content. It takes certain, usually very dark, scenes to bring the issues out. I played ~8 games (52 hours over 2 weeks) on this set before realizing something wasn't right. I noticed it in a few games, but naturally blamed the games until I decided to do some tests. It's really bad in some dark horror games but basically unnoticeable in most games, and that's why I think many won't ever notice it.
My tests are the submarine ride at the very start of Bioshock 1 Remastered and some of the later dark levels in Amnesia: A Machine for Pigs (which is the worst offender i've found). Its a night/day difference when it's working as intended in these. It ruined my Amnesia experience, as I thought the horrible posturization effect was just poor design choice, I was googling trying to find ways to get rid of it but no one had the same issue. Game looks great in YCbCr444....