This happens if you set the RadWindow implementation (for example MainWindow.xaml) as a StartupUri of the application in the App.xaml file. To avoid this and show the window properly, remove the StartupUri and override the OnStartup() method of App.xaml.cs file. Then manually call the Show() method of the window.
This approach is shown in the following help article. Please give this approach a try and let me know how it goes.
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