Shell: ContentTemplate not firing Appearing on Pages

I'm experimenting with Shell. In this example I have some pages set to ContentTemplate:

    <FlyoutItem Route="main"
            Title="Main"
            FlyoutDisplayOptions="AsMultipleItems">

    <Tab Title="Animals"
                 Route="animals"
                 Icon="tab_1.png">

                    <ShellContent Route="tags"
                                  Style="{StaticResource MainShell}"
                                  Title="Topics"
                                  Icon="tab_a.png"
                                  ContentTemplate="{DataTemplate prayerviews:TopicPage}" />
                    <ShellContent Route="groups"
                                  Style="{StaticResource MainShell}"
                                  Title="All"
                                  Icon="tab_s.png"
                                  ContentTemplate="{DataTemplate prayerviews:AllPage}" />
    </Tab>

Both pages TopicPage and AllPage have Appearing events handled, but only TopicPage's gets fired.

How can I be notified that AllPage is showing so that I can load the expensive Model ?

Appearing is fired:

    public TopicPage() 
    {
        InitializeComponent();
        BindingContext = App.IoCContainer.GetInstance<TopicViewModel>();

        Appearing += async (sender, args) =>
        {
            await Refresh();
            base.OnAppearing();
        };
    }

Appearing is NOT firing:

    public AllPage()
    {
        InitializeComponent();
        BindingContext = App.IoCContainer.GetInstance<AllViewModel>();

        Appearing += async (sender, args) =>
        {
            await Refresh();
            base.OnAppearing();
        };
    }