RadToolTip is rendered with an HTML table and respectively has some cell (TD) elements. In your case, I suppose, that you have some global CSS styles applied to all TD elements in your application - padding, margin, height etc., that are also applied to the TD elements of the ToolTIp and breaks it`s layout.
Try to disable all custom CSS in you application and check if the problem still exists. If it is gone, enable again your custom CSS and try to figure it out, which CSS rule is breaking ToolTip layout.
If you need further assistance, we will need a sample code project that represents the problem, in order to able to test it.
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