Group ensures proper query access by altering three types of queries: the group queries, the group_content queries and the entity query for any entity that can be grouped with access control. Entities that can be grouped without access control do not have their queries altered.
So what happens when you try to group group entities with access? Well, these query alters crash and burn is what happens. So we're no longer allowing it. If you want this functionality, I would highly recommend you check out the Subgroup module by yours truly or other, similar solutions.
Group offers you all the tools available to alter access over group entities, so there's no need to group groups to achieve that purpose. You can dig into how Subgroup uses a custom group permission calculator to figure out how this is done.
