This behavior occurs in a very specific setup and it cannot be isolated easily. Currently, it is reproducible in RadMap when you use RadToolTipService to set a tooltip to an item in the VisualizationLayer. If you hover the item and zoom in or out without moving the mouse (without hiding the tooltip), the tooltip popup is still displayed, but its content is empty.

To work this around, use the WPF's native ToolTipService. If you want to be consistent with Telerik's theming, you can use the RadToolTipContentView as the ToolTipService.ToolTip value.