Chosen Solution
Along the top of a black iPhone 6/6S/7/8 (and their plus equivalents) there is a small white sticker under the ambient light sensor that filters light. This only exists on the black screens. I always transfer it over when doing a repair, but the odd time the area where the original is damaged has either caused it to go missing or has torn it causing it useless. Am I able to get a replacement for this small white piece of (seemingly) paper? Without it, an active phone call placed to the ear does not black out the screen - resulting in buttons being pushed on the screen during a call.
I think you are getting confused with the ambient light sensor and the proximity sensor. The proximity sensor is there to detect when the phone is up against your face, putting the screen to sleep. The ambient light sensor is used to adjust the brightness of the screen in different lighting scenarios. This is the one that has the filter paper. I have never really noticed if this makes a difference if it is there or not on the black screens. If you think the ambient light sensor is not working correctly due to the filter missing you could just cut a piece of filter paper to pop in there. I have heard of people using a white marker but I am not sure of the results.
