I think that this may be a software problem, not a hardware problem at all. I have Nothing Phone (2a) and there is no such problems. And as you mentioned, the screen is OLED so you have to expect some kind of quality decrease but that should not be as much noticible as you showed on the video, Android 15 is running fine, there for sure fixed a lot of things with this update and getting it checked out is a good idea but who knows if they can help you at all. Do you have the latest NOS 3.0 or still running 2.6?