The Header property setting is replaced with a default value when the RadVirtualKeyboardWindowAutomationPeer is created. This happens on environments where the automation peers creation kicks-in. Like, on touch devices, when a screen narrator application is used, on visual elements inspection with some tools (like UISpy or Snoop) or any other automation testing that requires automation peers. 

In that case, instead of the user defined Header value, the window's header is replaced with "RadVirtualKeyboardWindow" string.

To work this around, disable the automation peers:

public partial class App : Application 
{ 
    public App() 
    { 
        AutomationManager.AutomationMode = AutomationMode.Disabled; 
        this.InitializeComponent(); 
    } 
}