This is expected behavior. The XmlHttpPanel uses ASP.NET client callbacks and web services to update its content. This means that there is no ViewState and the Controls tree is not created. Basically, the XmlHttpPanel retrieves the HTML content as a string, and pastes it on the client. If a postback occurs, the content will disappear - since it does not exist on the server.
If you plan on using server-side events, that require page postbacks or Ajax calls, then you should switch to using Ajax Panel instead.
More information about RadXmlHttpPanel can be found on our online documentation
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