I have found a bug in PlexAmp on Android 3.4.1. When you connect to a Chromecast (in my case Chromecast Audio) and then connect your Bluetooth headphones (I do it via NFC, WH-1000XM3) and after that disconnect from the Chromecast the audio will play on the phones speakers and not on the Bluetooth headphones. After disconnecting the headphones and reconnecting them the audio will play on the Bluetooth headphones. I’m using a OnePlus 6 btw and running a Plex Server on Ubuntu Server 20.04 LTS.
Regards Inspirer