Iphone screen wont turn off consistently after 20 seconds when set in automatic mode.

I have standby mode active on my iPhone at night when charging. I have it set on automatic mode where the screen shuts off after 20 seconds. The automatic mode seems very inconsistent. Some nights the screen will shut off and can come back on and shut off again after the set time automatically. Some nights when still in the same mode and haven’t change any settings the screen will stay on all night and won’t stuff off at all. The only way to to shut it off is to manually do it and manually turn back on again. Don’t know if anyone else has the same issues or has a solution. I know you can change the setting but I like the auto mode when it works so you don’t actually have to touch the phone.