I'm measuring Plex on my LAN against Plex on my LAN. Some of my clients accept 4K streams with no problems. Other clients buffer an unwatchable amount even on 480p from the same server.
My purpose of bringing up Netflix was only to show that the client devices that are choking on 480p from Plex are capable of handling 4K streams.
Sampling all the way down to 480p is definitely odd behavior on a LAN--can't say I've run into that. Have you had much help from the usual spots? I'd be curious to help troubleshoot this sometime, sounds like an interesting nut to try cracking, or is it something inherent to the clients with the way they're decoding?