I can’t play any kind of 4k content via the new preview app, it always converts it to 1080p. I have direct stream enabled and quality set to original. TV vizio M-50 Chipset SX7A(-1).
I had this exact issue with the previous app which I hopped would be fixed in an update.
In the app there is an option to send logging info yo your server. Play the file again without subtitles, let it play for 30 seconds. Get me the log from your server. Plex Media Server Logs | Plex Support