Yesterday:
I went to take shower (less than 20 mins). When I came back it was off.
I went to watch TV in my living room (an hour or so). When I came back it was off.
🤔 🤷♂️
See, that's what I mean. The phone(s) don't even have any sort of set pattern or time frame(s) when their "always on display" shuts down. But in your specific case, the display turned off after only 20 minutes? Really????
Apple needs to go back to the drawing board on their almighty "Always on display" and instead call it, "Randomly on or off display" instead. lol