The proximity sensor is in the center at the top of the mainboard, so that would be the place to check:[br]
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Sensor at the top: https://guide-images.cdn.ifixit.com/igi/BJS2eXrS6AgkYlSL.huge[br]
Another view with the rubber piece still on the sensor: https://guide-images.cdn.ifixit.com/igi/PdfnvdTSWAKsF2VW.huge[br]
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There is a rubber piece that goes over it and has a hole for the sensor. It's possible that the rubber piece got misaligned or something else is in front of the sensor. I would definitely wait until you have replacement screen adhesive before you check. Since it's on the mainboard, if nothing is obviously wrong then your only option might be to just get a new mainboard unfortunately.