Volume limiting itself

[AC Question]

I'm having trouble with the volume control on my oneplus 3 running android 7.`1.1 ( or latest oxygen os). Whenever it is connected to an aux device - headphones, car stereo, speakers. The volume starts limiting itself to around 80% and if the rocker is pressed to increase volume past this, the screen locks itself. And if the rocker is repeatedly pressed the phone keeps locking, makes an audible notification sound (different to what i have set for any other apps, calls or msgs) and the background for the lock and home screen changes to a default picture from the one i had set. This problem has persisted over multiple oxygen os updates.