Since update it forces 720p

Famous last words those…

Check the Dashboard Details during playback of one of these items.
Make sure you’re ‘Nearby and Direct’ - NOT Relay.
While I’ve never seen Relay in operation - this smells like something Relay would do.

and just in case…:

https://support.plex.tv/articles/216766168-accessing-a-server-through-relay/

Relay happens when a major malfunction occurs and Clients can’t access the server directly - for some reason.