MyTemplateResourceDictionary.xaml

<?xml version="1.0" encoding="utf-8"?>
<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:ContextFlyoutRepro"
    x:Class="ContextFlyoutRepro.MyTemplateResourceDictionary">

    <DataTemplate x:DataType="local:MyClass"
                  x:Key="MyTemplateDataTemplate">

        <StackPanel Background="Black"
                    Height="400"
                    Width="400">

            <Button Click="{x:Bind OpenWindow}"
                    Content="Click to open me on another window"/>

            <Button Content="Left mouse button for Flyout. Right mouse button for ContextFlyout">

                <Button.Flyout>

                    <Flyout>

                        <TextBlock Text="This opens normally"/>

                    </Flyout>

                </Button.Flyout>

                <Button.ContextFlyout>

                    <Flyout>

                        <TextBlock Text="Can you see me?"/>

                    </Flyout>

                </Button.ContextFlyout>

            </Button>

        </StackPanel>
        
    </DataTemplate>

</ResourceDictionary>