I updated 2 days ago my S20 SM-G981B to Android 12, official, non-beta. The phone started misbehaving badly:
Anything I could do to troubleshoot or get back to normally working Android 11?
Of course, I have the latest firmware and latest versions of all apps (only from Google App Store and Galaxy Store).
The worst firmware I ever had.
I got almost the same problems on S20+, my battery went from 60 to 4% in less than 2 hours, everything is running slow, I hate the "extending" animation when you scroll to the end (I preferred the white line that shows up), mobile data is ultra slow...
Not sure what to tell you other than check your IMEI and search for a ROM that fits your IMEI and flash the phone through ODIN, I suggest this if your warranty is done.