Your code seems to be correct, I built a test demo based on it and it worked as expected. The snippet of the source you provided also seems correct and the tooltip is correctly initialized. That is why I cannot tell you what causes the issue until I debug the problem.
Would you please examine the attached demo and test it on your side? In case it works fine, modify it in order to replicate the issue, open a new support ticket and send it to me along with your explanations and I will do my best to help you resolve the problem.
the Telerik team
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