[Jeff Sayre]

bp_the_topic_poster_avatar() pulls the avatar of the people who post in a particular forum thread. It does not pull the overall group avatar.

Do all your users have unique avatars?

Are you on a dev site?

Log into WPMU’s backend as Site Admin and navigate to “BuddyPress > General Settings > Default User Avatar” and see if you have selected the Mystery Man avatar. If so, change it to something else and save your settings.