Proximity Sensor won't work, no matter what!

Hey Everybody!

I’m at my wits end here - i’m sitting here with 2 iphone 7 on which i have replaced the screen, just like a thousand other iphone 7’s.. I’m used to sensors not wanting to work on every screen, and sometimes you have to adjust the sensor - sometimes you need to either scratch off some of the “painting”, sometimes you have to make it a bit darker - standard stuff.. However, this time, something really is off.. No matter what i do, there is no life in the sensor - i’ve tried 10 different sensor flex, i’ve tried 10 different screens - including 100% original screens (pulled). In every case, the camera works, the speaker works, but the sensor is dead.. Completely.. I’ve also had the sensor flex installed loosely, without mounting it on the screen to test with open phone, still the same.. Before installing, i remove the battery, every time - also on the original repair.. Nothing looks burnt, as i’ve had them both under microsope, and nothing looks off either..

Both phones run IOS 13(.x.x) - software wipe has not helped either.. Has anyone experinced anything like this? Does anyone have a clue as to what i can do? Board-level repairs are also welcome, if anyone can tell me where to start looking!

Many thanks in advance!