So, a rights issue, then... Had my "share" of these when configuring network shares
As far as I know, when PM accesses a read-only folder (with no write access rights), it behaves similar to when you set the option to "Allow modification of files on Camera Disks (Not Recommended!)" disabled (default). So it presumably stores any edits (tags, rating,...) in local memory or cache only, forgetting them upon restart. Which is IMO nice behaviour, as it still allows you to make a temporary edit to say select a few files to copy elsewhere, without touching the archive. Myself I am slowly migrating to having the NAS mounted over a read-only share normally, while copying photos to it via rsync script with write credentials. That way I can hopefuly mitigate a finder crash or user error from wiping important files from the backup.
BTW, out of curiosity, why are you accessing it over NFS in OS X? Is it in any way better than SMB or AFP on the Mac?