Want to make only secondary usergroup show up on member list
 

Per a client's request, I disabled the display of all members on memberlist.php, except for one specific usergroup. The problem seems to be that the specific usergroup is a secondary group. No one in that secondary group is being displayed on the member list, even though I clicked 'Yes' for 'Viewable on the Memberlist'.

Is this by design, and if so, is there a way to change it?