The same thing was happening to me as i was researching the answer. None of the solutions such as Shut down Mac, hold Command option shift and power button or command option P R and holding power button for 20 sec helped.. meanwhile my mac was on charge and about 40% full... as i was researching, it suddenly switched to the external display (which is the problem for me) by itself (it was plugged in all the time) once the charge reached 50%... its strange because i haven't read about it on any apple website or support forum and i checked all of them. this is how i resolved it . macbook pro 2016 touchbar 13inch.