Ok, so exact same issue then…
Can you show:
nsenter --mount=/run/snapd/ns/lxd.mnt grep shmounts /proc/self/mountinfo
journalctl -u snap.lxd.daemon -n 300
To fix that particular one you’ll actually need:
nsenter --mount=/run/snapd/ns/lxd.mnt umount /var/snap/lxd/common/shmounts/storage-pools/zfs1/containers/docs/