group_list in ldap_fluff returns a list of lowercase LDAP GIDs. https://github.com/theforeman/ldap_fluff/blob/master/lib/ldap_fluff/generic_member_service.rb#L56
When foreman tries to run update_usergroups to refresh the external user groups, it will try to match these lowercase gids with external user group names. However, we don't enforce external user group names to be lowercase. If an external user group contains any capital letter, it will not be synced as it will not match the lowercase GID.