Having the same issue but I guess I found the problem.
Check the upper right corner of your display, hold the phone sideways and look if the screen comes out from the frame, that's the case for me. The upper right corner of my display sticks about 0.8mm out of my frame, this somehow f*cks up the proximity sensor.
If I press down the corner it works for a moment but the display slowly pops out again.
Didn't find a solution yet. I tried to glue it carefully with loctite, but after a day the screen is sticking out again. I don't know if this could be solved with a software update by decreasing the proximity sensors sensibility. Two other options I see are that Nokia let us disable the proximity sensor or let us recalibrate it with the faulty display.
Please reply if any of you are experiencing the same problem with screen popping out of frame.
PS. English isn't my native language so please don't blame me for any grammatical faults