I have had this issue many times now. Every time except this most recent, I have fixed the issue permanently by un-seating the sensor/camera flex cable, powering on the phone, and plugging it back in while the phone was powered on.
After this you could reset the phone, remove the battery, ext. and the issue would not persist.
Yesterday I repaired a customer's phone and the issue happened again. I did the same as I always have and it worked through multiple test resets but as of this afternoon the problem came back and disconnecting none of the peripheral devices from the logic board seems to have fixed it.
I have also attempted to force an overheat of the phone to no avail.