Hi, I'm using a z5 compact (E5803) and obviously having the same trouble you are experiencing... but i have a workaround: everytime i'm gonna connect a accesory (headset, car audio) I connect it as always, everything that is bad will be still bad, but... if i restart my phone with the accessory connected and Bluetooh still ON, before the rom loads completely the accessory reconnects again a voila! all the troubles will be gone!... pitifully, if I disconnect it again the troubles will be back. I tried with my BT headset (MDR100ABN), and my car audio (PIONEER) and confirm with both ones this workaround success, so... maybe you wanna give a try until SONY brings a proper fix to this.
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