MDI control box does not appear when MDI child forms are hidden istead of closed. This happens the second time you are trying to open a previously hidden form right after hiding another MDI child form.

Workaround:
 First, you should add the RadMDIControlsItemExtended class to your real solution and then add the following code snippet in the constructor of the parent MDI form:
this.rbMain.RibbonBarElement.ButtonsContainer.Children.RemoveAt(2);
RadMDIControlsItemExtended MDIBox = new RadMDIControlsItemExtended();
this.rbMain.RibbonBarElement.ButtonsContainer.Children.Add(MDIBox);
MDIBox.LayoutPropertyChanged();

RadMDIControlsItemExtended.cs:

using System;
using System.Collections.Generic;
using System.Text;
using Telerik.WinControls.UI;
using System.Windows.Forms;
using Telerik.WinControls;
using System.Reflection;

namespace StandardMdiApplication
{
    class RadMDIControlsItemExtended : RadMDIControlsItem
    {
        protected override void OnHostFormLayout()
        {
            base.OnHostFormLayout();

            FieldInfo fi = typeof(RadMDIControlsItem).GetField("hostForm", BindingFlags.NonPublic | BindingFlags.Instance);
            object o = fi.GetValue(this);
            Form hostForm = (Form)o;

            if (hostForm != null && hostForm.IsMdiContainer)
            {
                Form maximizedForm = null;
                foreach (Form form in hostForm.MdiChildren)
                {
                    if (form is ShapedForm)
                    {
                        foreach (Control mdiFormControls in form.Controls)
                        {
                            if (mdiFormControls is RadTitleBar)
                            {
                                mdiFormControls.Visible = form.WindowState != FormWindowState.Maximized;
                            }
                        }
                    }

                    if (form.WindowState == FormWindowState.Maximized && form.Visible && (maximizedForm == null || hostForm.ActiveMdiChild == form))
                    {
                        maximizedForm = form;
                        break;
                    }
                }

                if (maximizedForm == null)
                {
                    this.Visibility = ElementVisibility.Collapsed;
                    this.InvalidateMeasure();
                    return;
                }

                FormBorderStyle borderStyle = (maximizedForm is RadFormControlBase) ? ((RadFormControlBase)maximizedForm).FormBorderStyle : maximizedForm.FormBorderStyle;

                if (maximizedForm != null && maximizedForm.Visible && maximizedForm.ControlBox &&
                    borderStyle != FormBorderStyle.None &&
                    borderStyle != FormBorderStyle.SizableToolWindow &&
                    borderStyle != FormBorderStyle.FixedToolWindow)
                {
                    this.Visibility = ElementVisibility.Visible;
                    this.InvalidateMeasure();
                }
                else
                {
                    this.Visibility = ElementVisibility.Collapsed;
                    this.InvalidateMeasure();
                }
            }
        }
    }
}