When the audio source switched by long-pressing the Home button then selecting Bluetooth headphones during playback, the main dialogue audio doesn’t play, only the background audio plays. To fix it, must exit Plex, switch back to main audio source using same method, then switch again to Bluetooth source. If the source selected prior entering Plex player, everything works just fine.
This is only happening on Plex app, Netflix, YouTube, Amazon and all others don’t have this bug.