twest We had this problem with our backup servers, so we planned and installed a new backup server as part of the upgrade... but we still got stuck and had to unmount the /backups BTFRS partition on the backup servers and then decomission/delete them. So, we have the historical data preserved on those old servers, but started "fresh" with the new backup servers as part of the upgrade.
We also just ran into a weird "oops" where one client had backups on their own physical server (We do not recall why) and that came back to bite us yesterday since that backup role was missed in our v.12 upgrade. So, again, unmounted the partition, moved backup role to the EXT4 server and back in business.
Imagine all these small idiosyncratic things we each do/setup/utilize multiplied across many of us... no wonder there were so many revisions of v.12 in the first couple of weeks...
have you recovered now?