I noticed this issue on my Note 8 after I installed Sync.me. I disabled the CallerID feature (thinking maybe that’s preventing the screen from turning off during the call) and restarted my phone. The proximity sensor started working during phone calls, again. Then I reactivated the CallerID feature in the Sync.me app out of curiosity and tried another phone call and it still works. Not sure what the problem was, but maybe turning off the CallerID feature in a 3rd party app that you may have installed may do the trick (because it pops up during the call).