in App.xmal add FontFamily can't usable

.NET Core Version:.net core 3.1
Windows version: 1909
Does the bug reproduce also in WPF for .NET Framework 4.8?: No
Is this bug related specifically to tooling in Visual Studio (e.g. XAML Designer, Code editing, etc...)? If yes, please file the issue via the instructions here.
Security issues and bugs should be reported privately, learn more via our responsible disclosure guidelines.

Problem description: I use WPF(.net core)。 in app.xaml add FontFamily like this:
```
<FontFamily x:Key="Digital-7 Mono">
pack://application:,,,/IVFTempMonitor;component/Fonts/Digital7Mono-B1g5.ttf#Digital-7 Mono
</FontFamily>
```

in mainwindow.xaml use like this:

<Border
            Grid.Row="1"
            Grid.Column="0"
            Margin="3"
            Background="{StaticResource BootstrapCyan}">
            <TextBlock
                HorizontalAlignment="Center"
                VerticalAlignment="Center"
                FontFamily="{StaticResource 'Digital-7 Mono'}"
                FontSize="40"
                Foreground="White"
                Text="{Binding One}" />
</Border>

Actual behavior:

Expected behavior: but it's not usable.

when I use WPF(.net framework)，the code as the same. It's usable.

Minimal repro:

dotnet / wpf

in App.xmal add FontFamily can't usable #2709