Sensor at the top of the iPhone 8

I've never experienced that behavior with my device, but I have read of a couple of other users experiencing it. You can try creating a current backup of your phone and restoring to that backup. If that doesn't help, I suggest making a Genius Bar appointment at the nearest Apple Store and have the device checked. You can show the Genius what the device is doing, since it seems you can duplicate the behavior. It should not be doing this.

Sounds like you are asking about the Proximity Sensor. It will blank the screen when you place the phone up to your ear when in a phone call. Are you experiencing other behavior? Please describe what you are doing when it turns the screen off.

You can't turn it on and off.

You can't turn it off.

You're welcome.