To reproduce:

Type a word and then change the font. The RichTextEditor is not focused. When you click in it the font is restored.

Workaround: 
class MyRichtTextEditorRibbonbar : RichTextEditorRibbonBar
{
    public MyRichtTextEditorRibbonbar()
    {
        this.dropDownListFont.PopupClosed += DropDownListFont_PopupClosed;
    }

    private void DropDownListFont_PopupClosed(object sender, RadPopupClosedEventArgs args)
    {
        this.associatedRichTextEditor.Focus();
    }

  
}