Toolbars in Sheets merge with Container View

I'm using SwiftUI on macOS. The idea is to create a sheet, which has two buttons. One for cancellation and one for done actions. They are placed with their .confirmationAction and .confirmationAction placement options inside ToolbarItems.

When I now open the sheet, the container also adds this buttons to it's toolbar.

Is there something wrong with my code or is this considered an issue with SwiftUI?

Thank you for your assistance.
Tobias

Code Block 
//
//  ContentView.swift
//  Toolbar
//
//  Created by Tobias on 16.12.20.
//
import SwiftUI
struct ContentView: View {
    enum Selection: String {
        case First
    }
    @State private var selection: Selection? = .First
    var body: some View {
        NavigationView {
            List {
                NavigationLink(
                    destination: DetailsView(),
                    tag: Selection.First,
                    selection: $selection
                ) {
                    Text("First")
                }
            }
            .listStyle(SidebarListStyle())
        }
    }
}
struct DetailsView: View {
    @State private var displayAdd = false
    @State private var message = "Initial Message"
    var body: some View {
        Text(message)
            .toolbar {
                ToolbarItem(placement: .primaryAction) {
                    Button("Add") {
                        self.displayAdd = true
                    }
                }
            }
            .sheet(isPresented: $displayAdd) {
                SheetView(completion: didFinish)
            }
    }
    func didFinish(_ result: SheetView.Result) {
        displayAdd = false
        switch result {
            case .Success:
                message = "Success"
            default:
                message = "Cancelled"
        }
    }
}
struct SheetView: View {
    typealias Completion = (Result) -> Void
    enum Result {
        case Cancel
        case Success
    }
    private var completion: Completion
    init(completion: @escaping Completion) {
        self.completion = completion
    }
    var body: some View {
        Text("Modal Sheet")
            .toolbar {
                ToolbarItem(placement: .confirmationAction) {
                    Button("Done") {
                        completion(.Success)
                    }
                }
                ToolbarItem(placement: .destructiveAction) {
                    Button("Cancel") {
                        completion(.Cancel)
                    }
                }
            }
    }
}
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}