Screen doesn't turn off on iPhone 8

I'm having a little bit of trouble with my new iPhone 8, since the screen doesn't always turn off when notifications come up etc.

It's when the phone is locked and I either recieve notifications og just check the time, sometimes it turns off after a few seconds like it's supposed to, other times it doesn't. Just today I pressed the lock button to see the time, left my phone and then went out for about an hour. When I returned, the screen was still on, showing nothing but the lock screen. No notifications, so that couldn't be the reson that the screen was still on. It happens multiple times daily, and my colleauge and his wife has experinced the exact same thing. Anyone who has experienced the same problem? Or anyone who knows if the problem is in the phone or in the iOS 11?

I've tried turning the phone off - haven't reset the phone, but as two other people I know has the same problem, I don't think that it would fix it

Hope somebody's got a solution!

- Julie