Bug tracker
"topic_last_post_subject" not filled for new topics (fix completed in vcs)
So either that should not happen when editing the first post (only when replies are posted) or it should be done on new topic creation as well (last post can be the first post).
I would opt for later choice - but your call! Please make it consistent!
Comments / History
Also didn't find a code doing it in submit_post function.