Possibly related to bug 33640 , when creating a new item from additems.pl, you cannot add the item to an existing item group. 1.Create item groups 2.Add new item 3.Select Use existing item group and choose an existing group 4.Click Add item 5.View holdings table and item is not part of the selected item group.
Just tested this. Adding an item to an item group only works when choosing "Add/Move to an item group" from the holdings table. Otherwise it doesn't show up or have any function in OPAC (Aspen)