Hello,
For anyone still having problems with the ypbpr vs rgb probem.
I managed to get it stay on RGB. I tested restarting the computer, shutting down and opening again, disconnecting the usb-c cable that connects the computer to the monitor and connecting again, then waking from sleep. All worked.
I'm on 12.0 Beta (21A5284e). Display is a 4k Dell P2721Q connected with a USB-C to USB-C cable to a M1 Macbook Air baseline.
Search for DDC/CI option in the OSD of your monitor and turn it off. Let me know if that works for any of you!
Thanks!
P.S. If you want to replicate close to my own situation, before doing the procedure with the OSD I opened the lid of my Macbook and from Display Settings I changed to Mirror on the settings for my external display. Then I switched the color to P3 and back to the display's default color profile, and after that I did the OSD thing.
Seems like MacOS 12 Monterey fixed the issue for you so it stays on RGB.
Anyone else who upgraded to MacOS 12 can confirm this as well?