To reproduce:
- Add RadRichtextEditor to a form and assosite it with RichTextEditorRibbonBar
- Start the application
- You will notice that the font is Agency FB.
- If you click in the editor the font will change.

Workaround:
void Form1_Shown(object sender, EventArgs e)
{
    radRichTextEditor1.Focus();
    ribbonBar1.GetType().GetMethod("HandleFontStylePropertiesOnCurrentEditingStyleChanged", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance).Invoke(ribbonBar1,null);
}