if the phone is locked rollback protection will be enacted and you won’t be able to downgrade. The state is recorded in the vbmeta partition.
If the phone is unlocked it should accept the downgrade, but then re-locking, a vbmeta expecting a higher version could be a problem (I never tried, this is theory to me). So you should reset the vbmeta partition while unlocked.
I never had to do this myself, just read it in passing. Check those out:
- https://forum.xda-developers.com/t/advanced-bypassing-rollback-protection-to-downgrade-the-os.4511501/
-
/How to/ install /e/ R on a samsung galaxy s10e already running android 12 (see what’s written on vbmeta partition, the user obtained a vbmeta img from somewhere… I think
fastboot erasecould do it too but then it’s not my device)