https://wccftech.com/ios-14-5-will-fix-the-dim-glow-issue-that-occurred-at-low-brightness-on-iphone/

According to the iOS 14.5 release notes, Apple has finally resolved an issue where you might experience a dim glow on the display of your iPhone at low brightness.

iPhone users end up experiencing some sort of display issue which Apple always ends up patching. This time, iPhone 12 users specifically, have noticed a dim glow like effect appearing on their display when viewing something completely black at a low brightness level. It is visible when you look close enough and you cannot unsee it. It becomes more prominent when you start using dark mode more often.