Found an interesting bug in SwiftUI.
If you use a menu or anything that supports the .preferredColorScheme() modifier in a view right after the navigation bar, you'll get a barely noticeable gray stripe.
How to fix it:
- Don't use the modifier at all.
- Use a similar modifier .colorScheme() - but it will soon be removed in new iOS versions, but still works in the current ones.
- Wait for Apple to fix this bug.