Grouping fields with field group doesn't work on display of Drupal Commerce product in product variation
The Field Group module provides the ability to group on both the edit form and the display independently.

This means to add a field group to the form you have to do so on admin/structure/types/manage/CONTENTTYPE and to add a field group for display you do it on admin/structure/types/manage/CONTENTTYPE/display (this is for nodes, this will be in different places for other entity types).

If you want the group on both the form and the display you have to add it in both places.