Grainy Screen

I just got a new iphone 13 and the screen looks grainy almost darker and pixelated in the background, especially when the screen is lighter. It’s not super noticeable, but enough that it bothers me. After I restart the phone, it will be okay for a few hours and then the screen will go grainy again. I went into the apple store today to get it fixed, and it was fine for the rest of the day and then it went back. I haven’t dropped it or gotten any water on it, I’ve only had it for about a week. Please help!