Platform, device and operation system
What were you expecting to happen, and what actually happened
With the game open, waiting for the sleep timeout on the device causes the screen to sleep.
How often does this happen? When did it begin happening?
Steps to make it happen again
Open the game, don’t touch anything, wait for the device screen to sleep
How to fix:
Somewhere in your initialization code, add
Screen.sleepTimeout = SleepTimeout.NeverSleep;