Zones on invisible layers are not in the zone list.

An invisiable layer is any layer that is not selected and has the
visiblity flag set to off/false/not truthy. Thus, a visible layer
is either selected or has visible set to true.
Selected layer is always visible

If a layer was selected but not visible, zones in that layer would still
appear in the zones list and be editable, though only the editing helper
points would be visible. By making the selected layer visible, the
behavior becomes intuitive.