@funnylookinhat@fosstodon.org Have a look at your idmapd service on the client, this has caused me a lot of grief before. Check the domain name (it's just the NFS domain, can be anything you like, really, 'localdomain' is fine) is the same, or the uid => name resolution can get fucked up.
@funnylookinhat@fosstodon.org If the UIDs match on client and server you can just force v3, it should work just fine.