4A is contacting the Bluetooth-Manager API in the boot phase and for unknown reason, the BT hw controller and bluez are not initialized correctly at that time. As a consequence, no bluetooth controller is detected by 4A BT plugin and BT audio are then disabled without any consequence on the rest of the audio stack.

This only happens at boot time: as soon as things are initialized, a simple restart of 4A service is enough to enable BT audio and stream music through A2DP.