We will need more information on this in order to be able to help. Can you please open a new support ticket and send us a small sample project where the problem can be reproduced? From your explanations it seems like a CSS problem, however without being able to reproduce it locally, we cannot tell what the exact reason is - as you said it yourself, on our online demos the tooltips behave as expected.
Thank you in advance for your cooperation.
the Telerik team