For the iPhone 8, the screen does not dim or turn off at all when any app is open. This has been reported on Apple's support forums and one of the solutions tha

For the iPhone 8, the screen does not dim or turn off at all when any app is open. This has been reported on Apple's support forums and one of the solutions that worked for several devices, including my spouse's device, is - restart the phone. My first thought was that "iOS is the new Windows where the primary recommended solution to a problem is to restart the device."