Face down sleep mode Revvl 6 Pro

It definitely sounds like a proximity sensor thing. The proximity sensor is mainly known for turning the screen dark when you are on a call to prevent pushing buttons with your face. They are also used to turn off the screen when the phone is set down or put in a pocket to prevent other buttons from being pushed. There isn't a direct way to turn it off in settings because is not really recommended to do so, but a google search would get you some steps to do that. 

The only other setting that I think could affect your screen turning off when you set it down is the screen lock. Do you have a lock on your phone? Have you tried turning it off to see if the screen goes dark when set down?