I recently updated to One UI 3 (just got the update, because my phone is from Hong Kong) and I've noticed that the screen stopped turning off when I'm listening to calls and audio messages. I've checked the proximity sensor through dialing *#77692# in the phone app and, sure enough, it was turned off. I turned it on, went back, but the sensor still didn't work. I came back to the menu, where it's turned off again.
I'll provide a link to the screen rec of the bug: https://drive.google.com/file/d/11fd1w4MGI9-pJwnTCRNTrjXXVbaj39zj/view?usp=drivesdk
Model: SM-N9860 (Hong Kong)