1. Create RadRibbonForm and add code to start maximized 
2. Add status strip with some items on the ribbon form 
3. in the form constructor create a MDI child form, set it to start maximized and show it 
4. The result is that there is gap between the child form and the status strip

Workaround:
protected override void OnShown(EventArgs e)
{
    base.OnShown(e);
 
    this.WindowState = FormWindowState.Minimized;
    this.WindowState = FormWindowState.Maximized;
}