Canonical broke ZFS 0.8 in lxd 5.21; I doubt that’s the issue here, but it’s possible they broke other stuff too. Unfortunately, they also made it impossible to rollback from 5.21 to 5.20, because they modified the database schema and it’s irreversible.
Personally, I run standalone incus nodes. The risks of clustering breaking (and the difficulty of fixing it if it does) is too high for me, and outweighs the benefits of a single API view of all containers.
But worse is the auto-updating behaviour of snap. If you must run lxd, which means you have to run snap(*), then it’s critical to prevent snap auto-updating. At very least you should pin your software to a fixed branch:
snap refresh lxd --channel=5.21/stable
But if you migrate to incus, you can be free of snap completely - happy days!
(*) Actually, Debian packaged lxd 5.0 as native deb packages, but that was before the lxd license change, and it’s not going to be maintained going forward.