Same issue on new Phone. Everything works like charm on my galaxy s23 u. now with my magic v3 its capped at 480p. if i choose auto quality it aborts and im unable to play.
I’m not sure what I’m doing differently, but I can configure through 1080p through to 4k if I want. Here are some screen shots, Apple iPhone running 18.3.1 with latest Plex Server and Client.
Plex Server config to ensure remote stream bitrate is correct (I typically set this to 1080p 12Mbps)