Finally, this workaround seems to work:
- Export a backup to the “target” host
- Delete the VM
- Import it on the target host
incus export --instance-only manual-migration /var/lib/incus/backups/manual.tar.gz
incus rm manual-migration --force
incus import /var/lib/incus/backups/manual.tar.gz
But it would be nice to 1) be able to use incus move
and 2) somehow get rid of the “ghost” volumes.