I have analyzed your picture and I think the problem comes from a global CSS style set to TABLE, TR, TD, UL or LI element. But just looking at an image I could only to predict where the problem comes from.
I would suggest you to check for any global styles that could force the elements in the RadToolTip header to inherit some styles that breaks the layout.
Another option is to send us a project with the problem reproduced or to give us a live demo url where the problem exists, so we would be able to fix it.
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