Near field (proximity) sensor reacts slowly/hangs up calls

Continuing the discussion from Screen protector folie:

@jpjacobs asked this question in the screen protector thread.

Does the proximity sensor need the opening on the screen protector?
Could it make a difference?

What is the screen protector configuration of those who have had issues or at least some inconsistencies with the proximity sensor?

I too am using a 3M’s Vikuiti DQCT130 screen protector that although fits perfectly on the Fairphone’s screen, does not have an opening for the proximity sensor.
I am inclined to think it shouldn’t matter because I can’t seem to notice openings on screen protectors for other phones, but I certainly don’t know for sure.

What do you guys think?