Chiming in here - my situation was a sata card failure, which was replaced - however between the reboots the auto start pretty much killed the cache the same way as
https://forums.unraid.net/topic/94233-solved-rebuild-cache-pool/
So despite having no disk failure itself, im now trying to figure out how to rebuild the whole btrfs array to migrate my approximately 2TB of VM data that i have pinned to be exclusively on cache 😅
My guess is that, because no drives were really lost, i should be able to perform the recovery in the next 24 hours - but it isn't exactly a pleasant experience, needing to time sink into the recovery process due to such an issue.
My array is now with disabled auto-start, auto-start really should not be default behaviour, if there is a risk of permemenant data loss - which we would block from the UI and warn normally anyway.