When you have a RadComboBox inside a Tooltip and you scroll down to open the tooltip by clicking on the target element, the dropdown of the combo box does not take into consideration the scroll offset, it is positioned way off.
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?