The only place where we set the configuration upon install, and there it is set to uniqueMember.
What might have happened: if you clicked on “Count Groups” at some point, it kicks off the detection of the association attribute (when it is not set to default, which happens to be uniqueMember). If the more or the same amount of users (→ sorting) are returned for both attributes, memberUid will be used instead of uniqueMember. Could this be an explanation?