Xamarin表单-NavigationPage.TitleView在iOS上不起作用

我有一个简单的（文件->新项目）XF应用程序，不会显示NavigationPage.TitleViewiOS的内容。Android运行正常。我究竟做错了什么？

源代码可以在这里找到。

XAML

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:d="http://xamarin.com/schemas/2014/forms/design"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             mc:Ignorable="d"
             x:Class="SampleApp.MainPage">

    <NavigationPage.TitleView>
        <Grid BackgroundColor="Blue">
            <Grid.RowDefinitions>
                <RowDefinition Height="*" />
            </Grid.RowDefinitions>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="*" />
            </Grid.ColumnDefinitions>
            <Label Grid.Row="0" Grid.Column="0"
                   TextColor="Black"
                   Text="Page Title" />
        </Grid>
    </NavigationPage.TitleView>

    <StackLayout>
        <!-- Place new controls here -->
        <Label Text="Welcome to Xamarin.Forms!" 
               HorizontalOptions="Center"
               VerticalOptions="CenterAndExpand" />
    </StackLayout>

</ContentPage>

这是输出...

我也尝试了StackLayout，结果相同...

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:d="http://xamarin.com/schemas/2014/forms/design"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             mc:Ignorable="d"
             x:Class="SampleApp.MainPage">

    <NavigationPage.TitleView>
        <StackLayout BackgroundColor="Blue" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand">
            <Label Grid.Row="0" Grid.Column="0"
                   TextColor="Black"
                   Text="Page Title" />
        </StackLayout>
    </NavigationPage.TitleView>

    <StackLayout>
        <!-- Place new controls here -->
        <Label Text="Welcome to Xamarin.Forms!" 
               HorizontalOptions="Center"
               VerticalOptions="CenterAndExpand" />
    </StackLayout>

</ContentPage>