Forum auto subscribe doesn't auto-subscribe when duplicating/restoring a forum

Steps to reproduce:

1. Create a forum with auto subscribe on in a course with users
2. Check who is subscribed -> the users of the course will be
3. Duplicate the forum
4. Check who is subscribed to the copied forum

Expected result: I expected the copy to autosubscribe the existing users of the course too, just like a copied forum does.

Actual result: No users are subscribed to the copy