When reading an individual group, users within that group are returned by their username only.
When reading all groups, users within a given group are returned by the DN.
As the DN representation of a user is backend specific, this should be changed to match the individual group outcome, such that users are referred to by their unique usernames, not their DNs.
Query a group with a member in it, then query all groups. Note the difference in the returned "uniqueMember" element's format between the two responses.