Also having issues with this.
I've tried:
Backing up my cache data, formatting cache, rebuilding cache - Same issue
Downgrading back to 6.11 - Same issue
Downgrading, formatting cache as xfs, rebuilding cache - Same issue, still btrfs issues, not sure why..
Currently now upgrading back to 6.12, going to reformat cache as xfs again and rebuild by docker.img
Absolutely ridiculous..