Let me chime in on the "does my iPhone 12 have this issue?":
First, my backgound. I am a video and television guy that has years of color grading experience with DaVinci Resolve. Flanders Scientific LCD panels are not uncommon to me. Secondly, I have used every single iteration of iPhone including the first OLED versions (X, 11). Lastly, I have access to lots of iPhones because of my job. Like hundreds.
I've read every single post in this thread (324 posts). And my summary judgment is that all iPhone 12s have raised blacks in low screen brightness ranges, and this issue is not present in Xs and 11s.
All iPhone 12s.
As to why some people deny their iPhone 12s exhibit it?
-The problem is only shown in certain conditions
-some viewers aren't critical enough to notice
So in summary, all iPhone 12s technically have the issue, but if you don't see it--it's not an issue to you. It is, however, an issue for me.