There nothing broken in our DB and it may or may not be a vB3.8.X issue. But of course it is related to vB3.8.X because this is a vB3.8.X upgrade, LOL
On the other hand, I have read many posts on the Internet in other forums with a lot of people having issues when they migrate to Discourse, even from vB4. (this is normal) So, of course, I’m not the only person on the planet reporting migration bugs. No one likes migration. It’s boring for everyone. But I am OK with it, because I like Discourse. The guys who wrote it are artists! They are impressive.
Like I said, the “lame approach” which will work for sure is to simply dump our DB again, change all child forums we want to keep to be parent forums (manually) and do the migration again. I don’t want a ‘default’ topic for important child forums, and all of our main forums are children, not parents.
@michaelp if you don’t have any experience or interest in insuring this script works with the legacy releases of vB3.8.X, that is perfectly understandable. But on the other hand, I also have zillions of years coding, porting, migrating, building, fixing bugs, and I posted a bug report here at Meta Discourse related to this code. I love vB3.8 and still think, today, it is was a nicer forum, than vB4 and vB5. That is why I never upgraded to vB5 or vB5. We do not like vB4/5. 
If this is going to turn into a “well, it’s not vB4 and not vB5” discussion, then I am better off not to post any migration bug here (any more) and become more active after the migration is done.
Cheers.
I will keep a running accounting here and will drop off this discussion for now and just get it done: