The problem started with Plex for Android 9.16 when they added AV1 Direct Play. The issue is that AV1 is direct playing on devices without AV1 hardware and that’s why you get the black screen with audio only.
It works on your OnePlus 5 because your phone is capable of software decoding AV1.
I made a post about it over 2 months ago and I’m still waiting for Plex employees to respond:
