Why can't detect the message that Bluetooth headset is disconnected?

I have two iOS playback programs, one is openal playback; the other is audiounit playback; both are controlled by audiosession. but the audiounit APP of the play program, when I turn off the Bluetooth Wireless headset, can not detect the old device unavailable event.the both can detect the new device available,but audiounit&avaudiosession can not get the notification of AirPods turn off.