I foind that it was a positioning thing. I got so used to cheek presses on the screen in the old days, it became habit to hold the phone to my ear and then a slight amount away from my face closer to my mouth. With a device this large, you might not trip the proximity sensor. I have gotten better at it, and the screen stays off more.
The funny one is switching to speakerphone. When I set the device down,then reach towards the speaker button, I will pass in front of the sensor ans turn off the screen. You gotta sneak up to it from the bottom to push the button before the screen goes dark. Lol.