I examined your code but it seems to be correct and I cannot tell what could be causing the problem you report. Can you please prepare a simple, fully runnable reproduction demo which uses your setup and configuration? You do not have to make it complex, no need to put database, masterPage, UserControl, custom controls, etc - only the two grids bound to a fake programmatic datasource or XML and the way you have configured the tooltip. Once you succeed with this, open a new support ticket and send it to us along with very detailed step-by-step reproduction instructions and explanations of the desired and the actual values shown and once we get a better understanding on what is actually happening we will do our best to help.
the Telerik team