I think it has to do with the fact that the app does not actually support or even know you are using a bluetooth of ANYTHING for audio. The app is simply outputting audio and the OS mixer is redirecting it to bluetooth audio. The caveat is in order to use the mic on BT headsets the hands free audio profile needs to be DIRECTLY interacted with by the app. The app is only looking for a sound input from the OS mixer, and the mixer supports its built in mic and mic from the headset port and input.