When RAID1 saved the data but not the state

My home server is a ZimaBoard 2 running ZimaOS. The storage that matters is a RAID1 mirror, two 4 TB drives formatted BTRFS, holding a media library, backups, and personal archives, with the Docker application stack running off the same pool. It is a small setup, and for a long time it was a boring one, which is exactly what you want from storage. Then a migration went wrong, and it taught me more than a year of everything working ever did.