Group “Last Activity” not reflecting group gallery activity

Everything is working the way it should with Mediapress on my Buddypress site, members are able to post galleries in a group and the group activity feed shows the gallery activity… BUT on the groups index page (mysite.com/groups/), which orders by default based on most recently active, the groups do not recognize the media activity. For example, when I add a gallery to a group that was last active a week ago, the gallery appears just fine in the activity feed of the group, but the group itself on the group index page remains “Last active 1 week ago”, and it is not moved to the top of the list (sorted by most recently active).

How can I get the last active status of groups to recognize gallery creation or posting?