first time a new user views activity stream, it shows only group post updates

I'm running wpmu 2.9.2 with buddypress 1.2.3.

The problem: the first time a new user logs in, they see only their group's updates in the activity stream. If they reload the page, and after all subsequent logins they see the unfiltered site-wide activity stream.

(On login a user is directed to the site-wide activity stream -- the theme uses this as the default for the buddypress host. All users are automatically added to a group when they first sign up.)

Where is this redirect set? Why would first-time login redirect someplace different?

Any insight greatly appreciated.