Ok, mystery solved. It IS a bug.
The bug consists of a conflict between a member's individual MT status and the group's overall setting to moderate the first message of every topic. The group setting took precedence over the individual member's MT and the topic ended up unmoderated; only the first message was moderated.
The fix: a member's MT status should take precedence over a group setting to moderate the first message of every topic.
Messages are the sole opinion of the author, especially the fishy ones.