LXD 4.0.0 and ceph: can't restore from snapshot

Trying to restore a snapshot of a container, and I am unable to:

ubuntu@aa1-cptef101-n3:~$ lxc stop monitor-910-0000 --project <redacted>-staging2
ubuntu@aa1-cptef101-n3:~$ lxc restore --project <redacted>-staging2 monitor-910-0000 pre-siteyml
Error: Failed to run: rbd --id lxd --cluster ceph --pool rbd-lxc-aa0.a1f unmap container_<redacted>-staging2_monitor-910-0000: rbd: sysfs write failed
rbd: unmap failed: (16) Device or resource busy

I am able to take snapshots successfully, however.

@monstermunchkin can you take a quick look if you still have a Ceph setup around?

We have automated tests for the basic cases so this is likely some kind of corner case.