I see this also, under 1.5.2+, when trying to restore a Blackboard 5.5.1 course to Moodle. Moreover, the discussions are loaded into a forum, but no forum activity is created. If you then create a new Forum, you can go to the Forum button under Activities, and see there are two forums, one being the missing BB forum/discussion. But, the postings can't be read as I get 'Course Module ID was incorrect'. I hope Moodle->Moodle restores work better for forums!
Backup and restore
Restore does not restore labels unless I import as admin
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