It’s more than likely your eyes. That much brightness against a truly black background is going to make your eyes see a glow around the bright object. As others have said, in a dark room, set your phone to maximum brightness, turn it off, then turn it back on. You’ll see the same bloom around the Apple logo.
Great thing is, though, that you don’t have to bother with buying the 2024 iPad Pro either. If they’ve figured out how to push OLED to 1600 nits (or have a per-pixel lit micro-led) that glow is going to be just as annoying. Apple could always introduce a mode that brings up the “black” color to the “grey” of the current iPads for those that are bothered by it, but I’m guessing they wouldn’t.