Why did my map chart Interactive Layer change?

Why did my map chart Interactive Layer change?

When zooming in or out of a map chart, the Interactive Layer may be changed automatically.  If the Zoom Visibility settings are such that the designated Interactive Layer is no longer visible at some point, then Spotfire changes the Interactive Layer to be the topmost data layer in the list of available layers. This change was made to avoid confusion in the map charts.  If the Interactive Layer is no longer visible, the user can no longer interact with the map chart for marking. To prevent the program from changing the Interactive Layer, ensure that the Zoom Visibility settings are configured that it is always visible. TS-50962