ToolbarItems disappear when navigating to another page

I've built a Xamarin Forms Project (PCL) from template (VS 2017) with:
App.xaml.cs

MainPage = new NavigationPage(new MainPage());

MainPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:MyApp"
             x:Class="MyApp.MainPage">
    <ContentPage.ToolbarItems >
        <ToolbarItem Name="AppSettings" Order="Secondary" Priority="0" Activated="AppSettings_Activated" Text="App Settings"></ToolbarItem>
        <ToolbarItem Name="ProjectsAdministration" Order="Secondary" Priority="1" Text="Projects Administration"></ToolbarItem>
    </ContentPage.ToolbarItems>
    <ContentPage.Content>
        <Label Text="Welcome to Xamarin Forms!"
           VerticalOptions="Center"
           HorizontalOptions="Center" />
    </ContentPage.Content>
</ContentPage>

MainPage.xaml.cs

async private void AppSettings_Activated(object sender, EventArgs e)
        {
            await Navigation.PushAsync(new AppSettingsPage());
        }

When I navigate to AppSettingsPage the toolbar is still visible, but the ToolbarItems have disappeared. I would have ecpected them to be also visible. (???)