I think I found a possible solution.
I joined the Google Play beta testing : Join App Betas: Android (mobile), Android TV, Apple TV, iOS
My current ‘faulty’ ‘public’ Plex player version is 9.0.5.31871
The Plex player beta version is 9.1.0.31834 and is working fine with all the very large HVEC x265 remuxed movie files again with full Atmos / DTS-X sounds !
Note: Beta testing sounds potentially problematic too but it has fixed today’s problem. I will leave the beta testing once the public version reaches 9.1.0.31834 or something stable and functional