Have you tried calling your voicemail and then try covering the left corner of ear piece speaker to see if the proximity sensor is working.. If this does not work then you have a bad sensor and you need to contact CS about a possible replacement device.
If your other device had the same issue I would guess that there is a good chance that your issue is app related not device... The chance of having repeating devices to have the same exact issue is a slim to none unless you just have that luck... Try going into safe mode and see if you get any diffrent luck...
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Good stuff man. I'm trying safe mode now. My initial thought was app related but after fac data reset its still a problem. Do you know of any apps whose permissions include proximity sensor? Or perhaps something is slowing down the processes enough that it doesn't catch the sensor instruction in time before the screen times out?
Theres apps that can do this but none that jumps out, care to share apps that you got installed maybe the app can be isolated from the list...