HeaderedContentControl Templates binding with `<Run/>`

Describe the bug

When styling the HeaderTemplate or ContentTemplate of the HeaderedContentControl, if the text of any of them is set with the <Run/> blocks instead of directly as a string or a single binding, the text is not displayed.

Steps to Reproduce

This can be reproduced just with this little XAML snippet. Just create a HeaderedContentControl with custom styles for both the Header and the Content; one of them passing as a single string inline, and the other one as <Run/> blocks.

<Page
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <Page.Resources>
        <Style TargetType="controls:HeaderedContentControl">
            <Setter Property="HeaderTemplate">
                <Setter.Value>
                    <DataTemplate>
                        <TextBlock Text="{Binding}" Opacity="0.6"/>
                    </DataTemplate>
                </Setter.Value>
            </Setter>
            <Setter Property="ContentTemplate">
                <Setter.Value>
                    <DataTemplate>
                        <TextBlock Text="{Binding}"/>
                    </DataTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Page.Resources>

    <controls:HeaderedContentControl Header="Hello world" Margin="100 0">
        <controls:HeaderedContentControl.Content>
            <TextBlock>
                <Run Text="testing"/><Run Text="testing"/>
            </TextBlock>
        </controls:HeaderedContentControl.Content>
    </controls:HeaderedContentControl>

</Page>

The result of this snippet is the following (tried both in Visual Studio, and in XAML studio just in case):

Expected behavior

Expected behaviour would be the text to be displayed correctly, instead of displaying the object of the page.

Environment

Package Version(s): 7.0.1

Windows 10 Build Number: 19042:928 (20H2)
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [ ] October 2018 Update (17763)
- [ ] May 2019 Update (18362)
- [x] May 2020 Update (19041)
- [ ] Insider Build (build number: )

App min and target version:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [x] October 2018 Update (17763)
- [ ] May 2019 Update (18362)
- [x] May 2020 Update (19041)
- [ ] Insider Build (xxxxx)

Device form factor:
- [x] Desktop
- [ ] Xbox
- [ ] Surface Hub
- [ ] IoT

Visual Studio 
- [ ] 2017 (version: )
- [x] 2019 (version: ) 
- [ ] 2019 Preview (version: )