This was a false alarm! Due to my non-standard external drive setup - Manually moving /srv to an existing drive - setting storage.srv.uuid and storage.srv.old_uuid causes the new uuid to be mounted at /srv; which resulted in a double-mount in my case. That was weird. Just using one mount method worked, and I’ll now have to go tweak my scripts and instructions on getting this right.
The other post about hardening the nextcloud install in NextCloud data consistency issues and suggested workarounds stands, though, and I’ll leave that open.