Yes, this is kind of unexpected but happens because the ToolTip is set to the TabItem and the content of the TabItem is still part of it.
To get around that, you can set the tooltip to the header of the TabItem. If your header (pnl) is a visual item, you can attach the tooltip directly. Otherwise you may want to assign a HeaderTemplate which will contain the tooltip. In this template you can bind the tooltip to a property in the header object as well.
Does this work in your case?
the Telerik team