We don't currently regenerate all the existing mount units of installed snaps on snapd upgrade; maybe we should. This wasn't a problem so far only because our mountunit template almost never changes (the addition of After=zfs-mount.service is recent but previous changes to the generated template are 2/5/6 years old). But this is problematic.
I think the comment from James is spot on.
We don't currently regenerate all the existing mount units of installed snaps on snapd upgrade; maybe we should. This wasn't a problem so far only because our mountunit template almost never changes (the addition of After=zfs- mount.service is recent but previous changes to the generated template are 2/5/6 years old). But this is problematic.