Hey folks. I happened on this post trying to figure out why my monitor and/or M1 Mini would not sleep. Doubt this issue has anything at all to do with the YPbPr, but it got me looking since I recently replaced my MBP with a M1 Mini. My Dell P2715Q (connected to TB3 dock via DP cable) was indeed stuck in YPbPr mode and if I tried to change it in the OSD the screen went all crazy purple.
On a whim, I looked at the settings in the OSD for MST (under display) and it was set to OFF. I recalled having another issue a while back and I had to set this to Secondary. Can't remember what the heck that was, but it obviously selected either. So I changed it to Primary and as soon as I selected it the display changed and it's no longer in YPbPr mode. The colors on screen did shift slightly as well. I'll have to recalibrate the thing to make sure it's all good.
To be sure this stuck I rebooted and checked the OSD. Still holding on RGB.
Anyone out there care to try this as well? Hoping it helps this issue. Now I need to get back to figuring out the sleep issue.