The only possible reason I can think of is that the [X] button does not pass an argument, so if you check for that it could seem that the event is not fired, since the if() block would not be entered.
Also, if you have set the DestroyOnClose property to true and you have already opened and closed this RadWindow once the second time there will be no such client object and a new one with the default settings will be created, thus the handled will be lost. If that is the case either add the handler dynamically, or remove the DestroyOnClose property.
You can find attached a simple page that I used to test this behavior. Please examine it and see if there is any major difference with your setup.
In case you are able to reproduce this behavior with my page please send me the steps I need to take to see it in action so that I can investigate it.
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