Hello Steve White,
This happens because the ComboBox does not exist in the Page's Control Tree when the client-side object initiates the Items request.
So, in order to fix this the server-side ComboBox object must be added on the page during such requests on the same place in the Control Tree. Additionally, the z-index of the ComboBox's drop down must be increased, so that it appears above the Tooltip.
Please see the attached files as a reference to the needed modifications to the Page's code.
All the best,
the Telerik team