The ear speaker component contains a few different modules; the ambient light sensor, proximity sensor, ear speaker, top/front-facing microphone, and FaceID sensor. If just one of them were damaged during the transfer, it may cause a short which will cause the device to malfunction (freeze or reboot). In your case, based on ''when'' this symptom arises, I’m inclined to say that the FaceID sensor might have been damaged. It makes sense for iOS to attempt a FaceID scan during the unlock process, which is when your customer’s phone bricks.
Before deeming it a bad unit, I would definitely verify that the board connection is free of any dust or debris that may be obstructing it by re-seating the flex, and follow that with a hard reset. If you’re still in trouble, I would then test a known-good ear speaker component and check for the original symptom. If the known-good resolves the issue, the original part was damaged in some way unfortunately.