LDAP groups with empty group_base

We have a LDAP server where the groups are directly added to the root level.

So in order to find these groups, I tried it with an empty string for ‘group_base’ in my LDAP configuration.
I can find the LDAP groups and use them on a LDAP synchronizations page (/groups/testgroup1/-/ldap_group_links). So I selected ‘.mygroup1’ for ‘LDAP Group cn’ and Owner for ‘LDAP Access’ and added a synchronization.
But when I use ‘EE::Gitlab::Auth::LDAP::Sync::Group.execute_all_providers(group)’ I see the following message.

No group_base configured for ‘ldapsecondary’ provider and group link CN .mygroup1. Skipping

And the Users of the ‘.mygroup1’ ldap group have no access to the ‘testgroup1’ gitlab group.

Is this a bug or am I doing something wrong?