I tried to explain that this cannot be fixed via a software update. This is the nature of the display hardware. One thing Apple can do to mitigate people realizing the problem is to use dark gray background instead of complete black when dark mode is turned on.
There are obviously display calibration settings that can mitigate the problem while messing up with the colors of the display. As I said, you get frustrated when you realize it but you get use to it after a while. I'm using smartphones with OLED displays since 2013, and it was always there and it will always be there if you have an OLED display.
Note that the issue is more severe on iPhone X, Xs, and 11 Pro series. They somehow improved the problem on iPhone 12 series but it is still noticeable.