BuddyPress.org

Expected behavior as activity commenting and forum discussion are separate. (only a one way tie-in when a new post/reply is made and the content is sent to the activity stream). You can disable activity commenting on forum posts/topics and blogs postings.

I updated

https://wordpress.org/extend/plugins/buddypress-group-forum-extras/

which now includes an activity subplugin -> will pull in activity comments made to a topic post (no reply ability but at least someone can see the conversation from the stream)