My iPhone 6 has this nasty habit off switching off its screen too early. It’s been an issue in iOS 11 (probably even earlier) and unfortunately, it’s remains an issue in iOS 12.0.1.
I have auto-lock set to 5 minutes (the max setting in my list, apart from ‘never’). Regardless of that setting, after 2.5 minutes (yes, I timed it) the screen darkens and another 30 sec later it’s off entirely. I have never once been able to get this iPhone to respect my setting for auto-off. It happens regardless of battery power or charging.