In brand new iPhone 8 few users are facing issue with iphone screen ON time. When user presses the home button once, without any further action, the screen stays on till one presses the side button. This is happening even when the auto-lock set to 30 seconds. The screen still lit when the phone is locked. This is very annoying and battery consuming as iPhone 8 screen stays on indefinitely.
To fix the iphone 8 screen keeping itself on, Force a restart. Press and quickly release the volume up button and then the volume down button. Finally, press and hold the side button until you see the Apple logo. Once the phone is on again you should not see such issue again.
If this does not help it is recommend restoring the device via iTunes, and then re-adding your information, sometimes a clean OS install is all it takes to resolve these issues.