Proximity sensor not working after display replacement

Sometimes the quality of the glass on the replacement screen can play a role in how well the proximity sensor works. More specifically, it has to do with how dark or light the class is where the light is designed to come through just to the left of the ear speaker hole. In your case, it sounds like too much light is getting through, so perhaps the glass is not dark enough there. Coloring the glass with a layer of Sharpie ink will often fix things if this is the issue.

If that's not the issue, then it may be a problem with how they installed the bracket that holds the proximity sensor in place. If the bracket is out of place or even missing, then the sensor may be too far away from the glass and too much light may be getting into the area around the sensor.

Checking either of these things would involve taking the screen off again and disassembling the front camera/sensor flex cable area.

Of course, it is very possible you just got a faulty proximity sensor. In that case, I would just replace it with the the one from your broken screen.