BuddyPress.org

bp_get_the_topic_post_poster_name() will only work inside a bp_has_forum_topic_posts() loop (i.e. the loop to get all the posts for that particular forum topic). forums-loop.php uses the bp_has_forum_topics() loop instead (i.e. the loop to get the list of all the posts in that forum).

I don’t think there’s a built-in function to get what you want, unless bp_the_topic_last_poster_name() would be an acceptable substitute?