If I left-click on one of tab header, the related tab expand itself showing all the contained items.
But, if I expand the tab using the mouse right button, the tab expand itself but it appears blank!

Workaround:

private void radRibbonBar1_MouseDown(object sender, MouseEventArgs e)
{
    RibbonTab tab = this.radRibbonBar1.ElementTree.GetElementAtPoint(e.Location) as RibbonTab;
    if (tab!=null && e.Button== System.Windows.Forms.MouseButtons.Right)
    {
        tab.IsSelected = true;
    }
}