We continue to try and workaround the snap/zfs issue, the latest attempt is here and will be in LXD 4.24 (if not already cherry-picked into LXD 4.23 by @stgraber ).
As for what you can do about this now, it seems to be triggered when the LXD snap base package (core20) refreshes (from chatting with @stgraber) seemingly causing an issue with the ZFS tooling not recognising the existing mounts inside the snap mount namespace.
Rebooting fixes the issue as it clears the mount table inside the snap mount namespace and synchronises it with the host. The latest PR tries to go further in avoiding the issue by trying to avoid getting the ZFS tooling from executing mounts/unmounts by setting the mount mode to legacy.
As to why the snap mount namespace gets out of sync, this may well be a snap bug, but so far a reproducer has not been found.
Any way to manually sync the mounts in the snap namespace @stgraber ?