Steps to reproduce: 
1. Create Telerik WinForms Application and add two RadForm
2. Select one of forms and set the IsMdiContainer property to true 
3. Select the child form. Open the Properties window and set the following properties at design time: 
 - WindowState to Maximized 
 - ThemeName to Windows8 or any other theme 
4. Run the application and show the child form. The child form is cut off instead maximized. 

Workaround: 
1. Reset the WindowState property at design time
2. Subscribe to the Form`s Load event and set the WindowState property 
private void RadForm2_Load(object sender, EventArgs e)
{
    this.WindowState = FormWindowState.Maximized;
}