Roster > group membership cache not expiring in a timely manner

Things hang around longer than they should in roster2, for a seemingly arbitrary amount of time. Steps to reproduce:

1. Create a project site, add the "Roster" tool
2. View the "Roster" tool; notice nothing is populated under the "Groups" column
3. Go to "Site Info" and create a group and assign yourself as a member of the group
4. Go back to the "Roster" tool; notice that the group you previously created is listed under the "Groups" column for your user
5. Go back to "Site Info" and delete the group you previously created
6. Go back once again to "Roster"; notice the group you deleted is still listed in the "Groups" column for your user
7. Click "Export"; notice the group is still listed in the export file
8. Reset the tool; notice the group is still present in the table and export file
9. Try navigating to another tool, or another site, and then back to the "Roster" tool in the original site; notice the group is still hanging around in the table and the export file

Resetting the tool any number of times has no effect. If you wait an arbitrary amount of time, the cache might refresh itself, but I've not seen a pattern in the amount of time. If you create a new group again, it forces the cache in Roster to update, but you can repeat the same problem indefinitely.