If I leave the phone screen during a call, say to look something up in a message thread or in my browser, the screen often goes black when I try to reopen the phone screen from the top menu.
This is the proximity sensor sensing you have your hand close to it. It turns the screen off when on a call.
This is not a bug. This is working as intended.
The proximity sensor turns the screen off during a call thinking you are putting the phone up to your ear to prevent accidental touches from your face/cheek on the phone screen.
