I am afraid I was not able to quite understand your scenario. As far as I understand you have a RadComboBox in a RadToolTip control. When the RadComboBox dropdown is expanded and you scroll you are supposed to close it as explained in this KB article
Would you please examine the mentioned KB and in case it does not help, open a new support ticket and send a sample, reproduction project along with a detailed explanation of the problem?
the Telerik team