SwiftUI NavigationButton within navigationBarItems

As I told you in comments, It was a bug. But it get fixed and it's working now exactly as you expected since Beta 5, But remember, NavigationButton has changed to NavigationLink. So it would be like:

struct ContentView: View {
    var body: some View{
        NavigationView {
            Text("Hello world")
            .navigationBarTitle(Text("Title"))
            .navigationBarItems(trailing:
                NavigationLink(destination: TestView()) {
                    Text("Next")
                }
            )
        }
    }
}