Some have it worse than others, but all of them have it. I know on the one I have, it's a very distracting dark blotch when scrolling through any site with a white background (so all of them) or the Settings menu. I'm going to return it and wait a few months to see if the hardware will improve.
Neither my 6S Plus nor my wife's have anything like this. Pages/settings with all white show nothing but a nice, uniform display.