Have this issue as well. I reported this (FB11359136)
MauricevB
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I just checked the code from the first post, but it seems to work here without any change 🤔 Xcode 14.0 beta 6 (14A5294g) in the iPhone 12 Pro simulator.
It seems SwiftUI ignores the UINavigationBarAppearance settings and doesn't have an API for changing/removing the bottom border indeed. There is also no API for forcing SwiftUI to use UINavigationBarAppearance.
There only solution I found is removing the new iOS 16 API's (.toolbarBackground(..)) and use SwiftUI-Introspect to set the navigation appearance manually:
.introspectNavigationController { controller in
controller.navigationBar.compactAppearance = appearance
controller.navigationBar.standardAppearance = appearance
controller.navigationBar.scrollEdgeAppearance = appearance
controller.navigationBar.compactScrollEdgeAppearance = appearance
}