I tested the code you provided and I reproduced similar issue to the one shown on the video capture and the reason for this behavior is that there is not explicit width set and the OnAjaxUpdate event is used. This is expected behavior in such case with the current implememtation of the tooltip - the content of the tooltip is loaded through AJAX and the tooltip cannot "predict" the width it has to have when it is shown. Thus it shows at some coordinates and after that the resize takes place and as a result the position is not the correct.
This being said, you should set the Width property explicitly and to use the ContentScrolling property to the desired value in order to determine the behavior when the width should be bigger than the set. If you want the tooltip to resize accordingly you should set it to Default.
We are currently working on improving this behavior but for the time being some explicit size set is the best solution for the case.
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