If you navigate using the NavigateURL property of the RadTab, you actually redirect user to that specific page. Using the NavigateURL prevents the client and the server events (except for the OnClientTabSelecting client event). In this way, what happens is that you do not select that specific tab, but just click the link specified. If you then perform page reload in some manner, you will be forced to the default state of the control. You can consult this KB Article
, which explains how to persist the selected state using the client-side API.
The above directions are just general principles. It will be easier to test your specific scenario, if you give is the exact markup of your page.
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