Don't know why android phones proximity sensor doesn't works as good as iPhone. Never had that problem in any iPhone and for android it seems all the phone have this problem.
I also used to have problems with proximity sensor not waking up the screen during a call. It seemed to be completely random: sometimes it happened after only 1 min of talk time, other time after 5 minutes, other time it worked ok even after 20 minutes of talk time. I turned off "double tap to wake up" and I never had any problems since. I'm not sure it's not a coincidence, but it worked so far (10 days).
About the cut off calls:it's not the proximity sensor. For me that happened on any European firmware. I tried the original .55 Vodafone Germany (the one that the phone came with), .55 UK and .402 Germany. It happened randomly on all of them. I tried repair firmware, factory reset, re flash with FlashTool. Nothing worked. The ones that work flawlessly for me are TW firmwares. I tried both .69 TW and currently on .402 TW. No cut off calls, no problems, no overheating in 4K (about 10 minutes recording).
Don't know why android phones proximity sensor doesn't works as good as iPhone. Never had that problem in any iPhone and for android it seems all the phone have this problem.