I seem to have a conflict between CFS 1.8.0 and a plug-in called Post Type Switcher 1.2.
It manifests when updating a Field Group. When updating, it appears to change the Field Group post type into a plain Post type. This is my assumption because an edit page for a Post (with the same name as the Field Group) is displayed instead of the Field Group page. After this the Field Group is missing from the Field Group list, although its fields are still displayed in places previously defined.