Hi Regie S
This occurs for quite some time, with every Bluetooth headset device connected to my Android phone (Galaxy S9+).
Both regular headset / headphones, and car handsfree.
The call starts with the bluetooth connected and selected as the audio output device.
After some time (seconds / minutes) Teams switches to the phone audio, and the bluetooth option disappears from the audio output dropdown list.
This, of course, is accompanied by some time when you do not hear the meeting, and try to understand why nobody is talking. Very annoying.
Just to clarify - the bluetooth device is still connected to the phone and works with every other app, but not with teams.
p.s.
Maybe related: similar issue when casting my Android screen to TV. Other apps use the TV's audio, but Teams only uses the phone audio.
Regards,
Tom