MAUI - Weird footer and header when changing from NavigationPage to AppShell

public App()
{
    InitializeComponent();
    MainPage = new NavigationPage(new StartPage());
    Instance = this;
}

protected override async void OnStart()
{
    try
    {
        // Check settings, session ,etcetc... TODO
        await Task.Delay(3000);
        MainPage = new AppShell();
    }