This is less of a bug and more of a quirk that I think could be handled differently because at first I didn’t understand what was happening and it created several hundred duplicate objects as a result of unclear communication.
What I initially did:
I had selected about 40-50 groups (each group consisted of around 12 actors, give or take) and moved them into a sub-level to clear them out of the persistent. Upon doing this, UE4 presented me with a message stating that one of the objects I was moving was referenced by another object, and whether I would like to delete that object. My knee-jerk reaction was to click “No to all”, and in doing so I left around 500 copies of all my selected objects behind on the persistent level in addition to new copies on the target sub-level.
What I discovered:
UE4 was in fact letting me know that it was ready to delete the old group after moving the copy to the intended sub-level, but does so one object at a time. This of course is what happens when you delete a group normally without copying it to another location because the objects in a group all reference each other. However, since the intended action was something different (I intend to “move” objects, not “delete” them) the message was unexpected and confusing from a user point of view unless you already know what is going on.
Conclusion:
I think it might be better in some cases to acknowledge and delete the group reference first before deleting the individual objects in the group, or in some way better communicate to the user that the old group is being deleted after the new group has been copied. OR prevent duplicates from being created in the new sub-level when the user selects “No” or “No to all”.