it appears you are changing the zoom properties of the map layer itself and not the heatmap layer, I replicated the behavior you are experiencing that way. I'd suggest resetting the map layer to the defaults and instead manipulate the zoom properties of the heatmap Layer itself. You can find these in the properties of the layer under the Layer Options section. By default the map should zoom to an area encompassing the limited expanse of "heat" on your map.
Hopefully that makes sense, let us know if not.