I have the same monitor and the 5700XT Red Devil
Same issues as yourself on DP and not on HDMI, this is a driver issue I think as I can work fine on 20.9.1
I have tried rebuilds, clean driver installs, and nothing is working, if I use 20.9.1 and earlier it all works, if I use 20.11 or 20.12 then I get the resolution issue and I have to reboot or pull cables from the monitor and its annoying now.
I have tried all sorts and I can't get the issue to resolve on the latest driver.
If I lock the machine, the monitor will go to sleep after 5 mins, come back 20 mins later and I get the same resolution issue.
I will keep checking the thread in case you find a resolution for this, I have raised bug reports for months via the driver but every new one doesn't seem to resolve the issue :(
I've tried the usual with the enhanced sync off, the HDCP off etc in the overrides. Not sure what the issue is, wondering if there is a BIOS fix for the card but can't see anything on the Red Devil site for a newer BIOS.
UPDATE:
Grabbed the 20.12.1 drivers, turned off Freesync via the monitors OSD and turned off freesync in the AMD drivers and now it will wake sometimes without he resolution issue but you do get the black screen issue! Still have to turn the monitor off to sort it. Will stick on the 20.9.1 for the foreseeable I think and try again in a few months.