Expected behaviour currently is that it can take up to 3 seconds to unlock during call using proximity sensor.

What happened for me is i am pressing power button too quick instead of waiting the 2-3 seconds for proximity unlock.
Only to then see the screen unlocked for a split second before my power press belatedly sends it back to black.

If it does not unlock at all after the 3 seconds, try a triple power button press to disable the proximity sensor temporarily. Learned that on TJC.

To test if proximity sensor hw is ok, just type in dialer for CSD Tool.

It was also theorized that it gets worse with uptime, so to not reboot the phone but restart sensors:
I was suggested to try which supposedly slows down interaction with the phone. So i only did temporarily.

Also mcetool is worth a try.
I tried all above and at some point i thought the problem is gone, but i am not sure anymore if i just learned to wait the 3 seconds out