I recently tried to get up to the iPhone X repair, and noticed some strange behaviour:
After every exchange of the Dislplay the ambient light sensor gets deactivated. It’s not faulty, defect or anything – it just gets disabled. The option is still shown, but the sensor does no response.
This also applies when exchanging displays (by displays i mean displays, not the sensors, as they are bound to face-id) from one X to the other. If you install the original display, the sensor works again. As if apple implemented an internal check and displays are bound to their Mainboard.
After doing some research i’ve found that other service provider are having the same issues.. Totally strange that nobody ever posted anything about this.