FYI, I am using FieldGroup 7.x-1.1+65-dev on Drupal 7.19. This note pertains to my experience with a content type's edit form.
After much trial-and-error, I have ascertained the following:
- Set up a top-level horizontal tab group "TOP". Then set up child horizontal tab items "T1", T2", and "T3" -- i.e., these are children of TOP. Add a mix of node fields and fieldsets containing node fields that are nested among T1, T2, and T3.
- All is fine and dandy as long as only node fields appear inside TOP. However, if the "title" node module element is added inside a fieldset that itself is a child of one of the horizontal tab items, say T2, then all is not well.
- What results is that ONLY the T2 horizontal tab item appears as a tab within TOP. On the other hand, T1 and T3 appear as FIELDSETS within TOP.
- Removal of the fieldset containing the "title" node module element plus the "title" itself from TOP puts everything back the way it should be. I.e., now T1, T2, and T3 display as expected.