Screen stays on if I am playing YouTube Music - What am I doing wrong?

[PaulQ]

Situation: Sitting at a sandwich shop. Playing YouTube music. Bluetooth headphones. My face out of range (it's about 18" away to my side). If I pick up the phone and it wakes up, look at the YouTube Music screen (do whatever) then place it back down next to me... the screen stays on.

If I put it down with YouTube Music minimized, just the home screen showing, it will shut down the screen in 15 seconds like normal.

I can't find anything in YouTube Music settings.

Am I doing something wrong or is this a glitch? Is it sensing something?