Swift: How to change DetailView depending on SideBar Selection State with SwiftUI 4?

Context

I have a problem with the new SwiftUI SideBar / NavigationLink concept. I have created a simple 2 Column NavigationSplitView with a SideBar and a DetailView.

Once the App is launched, the preselected Timeline RootView appears as the DetailView. However, when I select a different SideBar Item, the DetailView does not change accordingly.

Code

struct SideBar: View {
    @State private var navigationItem: NavigationItem? = .timeline
    
    var body: some View {
        NavigationSplitView {
            List(NavigationItem.allCases, id: \.self, selection: $navigationItem) { navigationItem in
                NavigationLink(value: navigationItem) {
                    Label(navigationItem.name, systemImage: navigationItem.symbol)
                }
            }
        } detail: {
            if let safeNavigationItem = navigationItem {
                safeNavigationItem.rootView
            } else {
                Text(String(localized: "select.an.option", defaultValue: "Select an Option"))
            }
        }
    }
}

Question

Do you have any idea how I can solve this issue? Thanks a lot for your support in advance.

Note: Just ran it on macOS, it is working there. However, still not working on iPadOS.