When you create a closed group it adds a member by default "Grant: Manage all Groups", this member can't be seen in the list of members but it permits those with the Manage all groups grant membership in the group.
The grants on the pre-installed Admin role are not editable so all Admin Role profiles have this grant.
It appears the problem is a group owner can delete the "Grant: Manage all Groups"
This appears to be a logical gap in the processing as it could create a scenario where all the owners exit the company for example and there would be no direct way to manage the groups.
Certainly I can see some cumbersome workarounds an admin could use to get around this limit but it seems to directly defeat the what I would expect the purpose of the grant "Manage all Groups" to do.