I’m sad to say I’m seeing the same issue. Not quite as severe, but very visible at low brightness and in a dark room. It isn’t noticeable to me at normal brightness or during the day. I’ve tried multiple reboots and it absolutely stays exactly the same after a reboot.
I don’t have another phone handy at the moment to take a picture of the issue, but I’ll take one tomorrow.
What’s strange is I’m only seeing this at low brightness on a grey background. Specifically, I first noticed it on the Gmail dark mode grey background. I took a screenshot of this grey and I can consistently replicate the issue. And yet, it does not occur for me with a black background at all. This is not what I would expect with burn-in normally, but I’m no expert.
I’m watching this thread. I’ll be eager to see what your resolution is. I’ll give my phone a few days and see what happens from here with this issue—whether it gets better or worse and then take it up with Apple Support.