RichTextBox serialization added "{}" when the first char in paragraph is "{"

namespace TestRichTextBox

{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            string str = FlowDocumentToString(RichBox.Document);
            xamlStringBox.Text = str;

        }
        private string FlowDocumentToString(FlowDocument doc)
        {
            var settings = new XmlWriterSettings { Indent = true, OmitXmlDeclaration = true };

            StringBuilder sb = new StringBuilder();
            var dm = new XamlDesignerSerializationManager(XmlWriter.Create(sb, settings));
            dm.XamlWriterMode = XamlWriterMode.Expression;

            XamlWriter.Save(doc, dm);

            return sb.ToString();
        }
    }
}

<Window x:Class="TestRichTextBox.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition/>
        </Grid.RowDefinitions>
        <Grid HorizontalAlignment="Left" Height="300" VerticalAlignment="Top" Width="483" Grid.RowSpan="2" Margin="10,20,0,0">
            <TextBox Name="xamlStringBox" HorizontalAlignment="Left" Height="152" Margin="0,138,0,0" TextWrapping="Wrap" Text="" VerticalAlignment="Top" Width="393"/>
            <Button Content="Button" HorizontalAlignment="Left" Margin="398,225,0,0" VerticalAlignment="Top" Width="75" Click="Button_Click"/>
        </Grid>
        <RichTextBox Name="RichBox" HorizontalAlignment="Left" Height="127" Margin="10,10,0,0" VerticalAlignment="Top" Width="393">
            <FlowDocument>
                <Paragraph>
                    <Run Text=""/>
                </Paragraph>
            </FlowDocument>
        </RichTextBox>
    </Grid>
</Window>

Run the sample code and type { in the RichBox at first char, Click the button, the xamlStringBox will show the serialized FlowDocument.  The paragraph shows:

 <Paragraph>{}{</Paragraph>

Is this a bug? Is there any walk-around?

Thanks