why does my screen turn off when i cover my front camera/face id?

The phone is using the proximity sensor and the screen is purposely designed to go dark when the proximity sensor is engaged. There is nothing wrong with the phone as it is doing exactly what it was designed to do. Just like when you answer a phone call and put the phone to your ear, the screen goes dark.

That is a feature that disables your screen when you hold the phone to your face, so contact with your face does not “tap” buttons on the screen. If you cover the proximity sensor (it’s a tiny red dot at the top of the screen) it is what triggers this feature.