To reproduce:
-add 2 RadRichTextBoxes and use the following code:
public Form1()
{
    InitializeComponent();
    Load += Form1_Load;
}

void SetDefaults(RadRichTextBox rtfbox)
{
    StyleDefinition style = rtfbox.Document.DefaultStyleSettings;
    style.SetPropertyValue(Span.FontFamilyProperty, "Microsoft Sans Serif");
    style.SetPropertyValue(Span.FontSizeProperty, (float)Math.Round(Unit.PointToDip(11), 1));
    rtfbox.Document.Selection.SelectAll();
    rtfbox.ChangeFontFamily("Microsoft Sans Serif");
    rtfbox.ChangeFontSize((float)Math.Round(Unit.PointToDip(11), 1));
}

void Form1_Load(object sender, EventArgs e)
{
    SetDefaults(rtf1);
    SetDefaults(rtf2);
}

private void rtf1_CurrentEditingStyleChanged(object sender, EventArgs e)
{
    radLabel1.Text = "Current font: " + (string)rtf1.CurrentEditingStyle.GetPropertyValue(Span.FontFamilyProperty);
    float fontSize = (float)rtf1.CurrentEditingStyle.GetPropertyValue(Span.FontSizeProperty);
    fontSize = (float)Math.Round(Unit.DipToPoint(fontSize), 1);
    radLabel2.Text = "Current fontsize: " + fontSize.ToString();
}

private void rtf2_CurrentEditingStyleChanged(object sender, EventArgs e)
{
    radLabel3.Text = "Current font: " + (string)rtf2.CurrentEditingStyle.GetPropertyValue(Span.FontFamilyProperty);
    float fontSize = (float)rtf2.CurrentEditingStyle.GetPropertyValue(Span.FontSizeProperty);
    fontSize = (float)Math.Round(Unit.DipToPoint(fontSize), 1);
    radLabel4.Text = "Current fontsize: " + fontSize.ToString();
}

private void radButton1_Click(object sender, EventArgs e)
{
    RtfFormatProvider rtfFormatProvider = new RtfFormatProvider();
    RadDocument doc = rtfFormatProvider.Import(File.ReadAllText("RadRichTextBox.rtf"));
    rtf2.Document = doc;
}

private void radButton2_Click(object sender, EventArgs e)
{
    RtfFormatProvider rtfProv1 = new RtfFormatProvider();
    SaveFileDialog saveDialog = new SaveFileDialog();
    saveDialog.DefaultExt = ".rtf";
    saveDialog.Filter = "Documents|*.rtf";
    DialogResult dialogResult = saveDialog.ShowDialog();
    string exp=string.Empty ;
    if (dialogResult == System.Windows.Forms.DialogResult.OK)
    {
        //use "RadRichTextBox.rtf"
        using (Stream output = saveDialog.OpenFile())
        {
            rtfProv1.Export(rtf1.Document, output);
            exp = rtfProv1.Export(rtf1.Document);
        }
    }
    
    RtfFormatProvider rtfProv2 = new RtfFormatProvider();
    rtf2.Document = rtfProv2.Import(exp);
}

Steps to perform:
1. Write some text in the fisrt RadRichTextBox and click the button for exporting. As a result the .rtf file content is correct (considering the desired Font - "Microsoft Sans Serif").
2.The imported content (from the exported .rtf file) in the second RadRichTextBox has incorrect Font: the default Calibri Font.

Resolution: 
This issue is addressed in the new version of the control - RadRichTextEditor. Please use the new control instead the RadRichTextBox.