I FOUND IT...after like 3 weeks of turning the entire project upside down and testing random things, fixing other bugs that weren't related, looking at compatibility stuff etc. It was a old objc UIColor override from 2013, which in hindsight makes sense sort-of, but it was just weird that it had worked up until now so I didn't find it right away (the project is over 500 files, with multiple extensions of other things as well, in objc and also swift) This is how it looked, if anyone stumbles across this weird bug and has the same issue: @implementation UIColor (TableViewBackgroundColor) +(UIColor*) groupTableViewBackgroundColor { return [TVColor TVTableViewBackgroundColor]; } @end Which must end up corrupt the internal color resolution on iOS 26.0+ somehow. Fix was to remove it/comment it out.

A few corrections to the description: Deprecated selectedImageTintColor property (returns nil when standardAppearance is set) Actually, it is deprecated in the code-behind, but still used in the Storyboard. It does work for my test project, just not my older project. And the fact that it was nil was probably because it was not set, although even if it was, it would be ignored as I've tried it multiple times. Tab bar selected item displays system default colors (gray in light mode, white in dark mode) I in light mode, the selected icon has a white outline/text (it is not filled, so the insides are transparent) then there is a grayed circle around the selected icon while the whole tab bar has a white translucent background. The unselected icons are have a black outline/text. In dark mode the selected has a light gray outline/text, with a slightly darker gray circle on the even darker gray background. The other icons/text are white. I just assume this is some defaults, or maybe in the code there is somewhere where this overrides the colors I want, but I can't seem to find it.