I found a reproducible issue in iOS 18+. When a SwiftUI TabView contains a Map (MKMapView), any custom UITabBarAppearance badge color is ignored while that tab is active. As soon as you switch to a tab without a Map, the custom badge color is applied again.

Minimal example:

struct ContentView: View {
    var body: some View {
        TabView {
            Tab {
                Map()
            } label: {
                Label("Map", systemImage: "map")
            }

            Tab {
                Text("Second")
            } label: {
                Label("Profile", systemImage: "person")
            }
            .badge(2)
        }
        .onAppear {
            let appearance = UITabBarAppearance()
            appearance.stackedLayoutAppearance.normal.badgeBackgroundColor = .black
            UITabBar.appearance().standardAppearance = appearance
            UITabBar.appearance().scrollEdgeAppearance = appearance
        }
    }
}

Looks like MapKit forces its own tab bar appearance. Anyone found a workaround?