I run two identical monitors that are AOC Q27G3XMN with Windows 11 and a GeForce RTX 4070 Super. Monitor 1 on the left, I use as the main monitor with the taskbar and mainly to play World of Tanks and Monitor 2 just to watch youtube etc. Both monitors have the same set up.
 
I initially noticed that monitor 1 would revert from RGB to YCbCr422 (Limited) upon a start up and not allow me to change it to back to RGB. Monitor 2 would always remain on RGB . At first if i removed and repalced the DP cable on monitor 1 it would allow me to select RGB but again revert to YCbCr422 the following day when restarted.
 
After seeing a post on here from a few years back, although way out of my knowledge range I reduced the monitors refresh rate from 180Hz to 170Hz. This did sort of work and both monitors would stay on RGB. It then became apparent that although monitor 2 would always stay on RGB 10bpc, monitor 1 only offered 8 bpc. I thought it was a DP cable issue so now both have the same cable as monitor 2 that always works fine (Silkland Vesa Certified Displayport Cable 2.1/2.0 2M, DP Cable Supports 4K@144Hz /165Hz /240Hz, 8K/10K/16K@60Hz, 40Gbps, HDR, ARC, DSC 1.2, G-Sync & Free-Sync, DP 2.1 for Gaming Monitor, Graphics Card).
 
Monitor 1 will now hold on RGB but only offers 8 bpc where as monitor 2 still holds on RGB with 10 bpc. If i revert the monitors back to 180Hz refresh rate, again monitor 1 flicks to YCbC2422 and monitor 2 untouched. If i select RGB for monitor 1 and click apply it just bounces back to YCbCr422. Until i select 170hz when monitor 1 again gets RGB 8 bpc and monitor 2 RGB 10 bpc
 
I cannot fathom why the two identical monitors act differently like this being the same, with the same settings?