Comment 7 for bug 1856871

FWIW snapd mounts and unmounts a squashfs on startup to determine whether the system can mount squashfs's. It does this using mount, and cleans up with umount -l. That is, it's not via systemd in this instance in particular.

I'm setting it as invalid for snapd, but if this behaviour is somehow tickling a bug and there's a workaround that could avoid it, let us know and we'd be happy to accommodate (set the bug task back to New so our triage picks it up).