Were you thinking about the June Patch?
15755 - When adding or removing members to or from a group/role, the subscribe or unsubscribe for all forums in the group is automatically carried out as long as there are 100 or fewer users for a group/role. When group/role membership is updated via a Job Scheduler task (for example, the LDAP sync job), all users are subscribed/unsubscribed and the 100-member limit is not enforced.
That may be part of it, but this room has way over 100 members. I had run across the "feature" in the previous version where adding a member to a group with more than 100 members did not subscribe the user to forums in the group. This is way more serious due to security concerns.