The person replied to the invitation, and was added to both the subgroup and main group.? Imagine my surprise weeks later when I found out.
it's not a bug, it's by design and it was changed to be that way in Feb 2019 (See ) I think at the request of an Enterprise group.? It's for that reason that we don't allow subgroup Mods to invite or add members within our group. It would be so much better if the (Direct) Add and Invite permissions were separated. It has been requested but there's been no movement on that to date.