Sharing this, as it might help other people:
When doing calls, my FP3 switched on the display every 30 seconds. Which is annoying, as you have to be very careful not to accidentially press the “end call button” with your cheek.
Finally I found the reason: It was caused by the function “Keep screen on while viewing” (off by default). This function can be activated in the Android settings, using the front camera to check if somebody is looking at the display.
Unfortunately this (useful) function is buggy (at least on my phone), causing the mentioned behaviour.
Workaround: Turn off “keep screen on while viewing”. Then the proximity sensor works like a charm again.