I always run my screen brightness at around 30-40% brightness for battery saving purposes and I still had it on my XS, 11 pro, and 12 pro. But hey, everyone has a different experience

Yeah that's the dreaded OLED screen burn in from long term usage. If you download a gray background off google images and blow it up to fill in your whole screen I guarantee you'll see burn in from the battery, cell bar signal, and clock icons at the top of your display since they are constantly illuminated for long periods of time (depending if you use dark mode often).

Definitely annoying but totally normal after a few years of use. My 12 pro max had it before I got my 14 pro.