Indeed, what you report is expected since you are using AJAX to load the content of the tooltip - when a partial update is done, the scroll position is lost and this is related to general ASP.NET and AJAX and it is not directly related to the tooltip but to how the things work (you can easily reproduce it by using a standard asp update panel and no RadControls at all).
What I suggest is to save the scroll position before the OnAjaxUpdate event e.g in hidden fields and when the content is loaded to reset it back. You can find more information about this and also other techniques on how to persists the scroll position on the net, e.g you can start from here:
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