Using NavigationStack in the details of a NavigationSplitView causes loss of navigation paths

In the executable code below I try to implement the main navigation view for an (iPadOS) app. The expected behaviour is as follows:

1. A NavigationSplitView is used to present a sidebar to the user containing the main categories of the app
2. If a user selects a category, in the detail view of the NavigationSplitView a NavigationStack is presented
3. The navigation stack allows users to interact with the specific part of the app associated with the category by pushing other views on the navigation stack
4. If a user switches goes to category A, pushes some views on A's NavigationStack, switches to category B and then switches back to category A, it is expected that the NavigationStack of category A contains all the views, the user pushed there earlier

However, (4) is not working. Every time a user selects another category in the sidebar, the navigation paths of the associated NavigationStack becomes empty. For example, if a user pushes three views on the view stack of category A, then uses the sidebar to select category B, the navigation paths of category A become empty and the NavigationStack of category B is displayed.

I kindly ask for help as I cannot find the issue in the code.

The following code demonstrates the issue:

import SwiftUI

/// Add this view to your app to test the behavior.
///
/// This view displays a sidebar with four entries (RootCategory enum).
/// Each detail view has a NavigationStack that referes to one of the Routing
/// state objects from the RootNavigation.
///
/// The goal is that users can switch the categories in the sidebar without losing
/// their navigation stack of their respective child view once they are going back
/// to a previously selected category.
///
/// However, this does not work because everytime a different categor is selected from
/// the sidebar, the navigation paths of the ChildView's routing is emptied. (see line 82).
struct RootNavigation: View {
    
    @StateObject var categoryOneRouting = Routing()
    @StateObject var categoryTwoRouting = Routing()
    @StateObject var categoryThreeRouting = Routing()
    @StateObject var categoryFourRouting = Routing()
    @State var navigationSelection: RootCategory? = .category1
    
    var body: some View {
        NavigationSplitView {
            List(RootCategory.allCases, selection: $navigationSelection) { category in
                NavigationLink(category.name, value: category)
            }
            .navigationTitle("App Title")
        } detail: {
            switch navigationSelection {
            case .category1:
                ChildView(title: RootCategory.category1.name)
                    .environmentObject(categoryOneRouting)
            case .category2:
                ChildView(title: RootCategory.category2.name)
                    .environmentObject(categoryTwoRouting)
            case .category3:
                ChildView(title: RootCategory.category3.name)
                    .environmentObject(categoryThreeRouting)
            case .category4:
                ChildView(title: RootCategory.category4.name)
                    .environmentObject(categoryFourRouting)
            case nil:
                Text("Selection does not exist")
            }
        }
    }
}

struct ChildView: View {
    
    let title: String
    
    @EnvironmentObject var routing: Routing
    
    var body: some View {
        NavigationStack(path: $routing.routes) {
            VStack {
                Text(title)
                    .navigationTitle(title)
                Button("Screen 1") {
                    routing.routes.append(.subview1)
                }
                Button("Screen 2") {
                    routing.routes.append(.subview2)
                }
                Button("Screen 3") {
                    routing.routes.append(.subview3)
                }
            }
            .navigationDestination(for: Route.self) { route in
                Text(route.rawValue)
            }
        }
        .buttonStyle(.borderedProminent)
    }
}

class Routing: ObservableObject {
    
    @Published var routes: [Route] = [] {
        willSet {
            // when selecting another category in the sidebar, the navigation paths of
            // the currently visible NavigationStack will be set to empty. Why and how
            // can I prevent it from losing the navigation paths of NavigationStacks
            // that disappear?
            if newValue.isEmpty {
                print("Navigation paths just got emptied")
            }
        }
    }
}

enum RootCategory: Int, CaseIterable, Identifiable {
    case category1
    case category2
    case category3
    case category4
    
    var id: Int { self.rawValue }
    
    var name: String {
        switch self {
        case .category1: return "Category 1"
        case .category2: return "Category 2"
        case .category3: return "Category 3"
        case .category4: return "Category 4"
        }
    }
}

enum Route: String {
    case subview1
    case subview2
    case subview3
}

#Preview {
    return RootNavigation()
}