This is 100% normal for oled screens but is really only noticeable on static gray screens under certain lighting conditions. It’s called the degree of “grey uniformity.”

People went nuts over this when the switch Oled came out (that has basically the same screen at a different size), since Nintendo’s “dark” theme is actually dark grey. So every time you turn the device on you see non uniform greys.

Again, in game you will never notice this. Someone more technical can probably explain why it happens though.