Hi,
So if I understand what you're saying, the order is confirmed, the subscription is added to the user in Akeeba Susbcriptions but the user group is not added to the user.
The plugin only handles the subscription of the user itself. The group being added to the user is something which is done by Akeeba Subscription and we don't have any control on that. And it's something that you configure on Akeeba Subscriptions end.
So in that case I would recommend that you contact the Akeeba Subscriptions support for help on that.