If you are opening RadWindow from the server (VIsibleOnPageLoad=true), this behavior is expected. When you navigate away from your parent page and then click the Back button, there is no trip to the server - the browser simply loads the cached state of the previous page. You will experience this behavior with any other control (for example you can try setting some text to a label).
To avoid that, I suggest to open the RadWindow control from the client by using the client-side API
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