Topics linked in Lessons still display and create orphans in DB if Forum deleted

I reported the problem in LSNBLDR-300, but this actually seems to be a Message Center problem, with Lessons exposing it.

If the instructor creates a Forum with 2 topics, (TopicA and TopicB), and links both topics through Lessons, has students post to both topics and then:
1) deletes TopicA - TopicA will show 'deleted' behind the title through Lessons
2) deletes the entire Forum - TopicB does not show 'deleted' behind the title through Lessons and continues to allow students to post through Lessons

To reproduce:
In a site with Lessons and Forums -
1) Click on Forums and create TopicA
2) Create TopicB
3) Click on Lessons and add TopicA and TopicB
4) Student roles: Post to topics A and B
5) Instructor: Verify postings are visible through Forums and Lessons
6) Click on Forums and delete TopicA
7) Click on Lessons - TopicA shows 'deleted'
8) Click on Forums and delete the entire Forum
9) Click on Lessons - TopicB does not display as deleted
10) Student role: Click on TopicB
11) Postings display - add another post

TopicB should have displayed 'deleted' just as TopicA did and not have allowed further postings.