Just picked up a Note 9 - I am on speakerphone 1-2 hours a day. Crazy how there are settings for SO many features, but not this. Just downloaded the Keep screen on app, will see how that works.
i got the dark screen when i am on call. then I came across solution about sensor. I used keypad to key in *#0*# and press the "sensor" to check if my sensor is blocked. It turn out that my sensor is blocked (the test screen was GREEN). and the root cause? my screen protector blocked my sensor even though it's supposed to be clear. Guess the sensor is too sensitive and detected it even though it's transparent. After I removed my screen protector, the issue went away!