On a migrated site, when assignments with attachments are brought over (or when imported from a previous site into a new one), the attachments retain their original id. If the attachment came from the user's local system, then it gets shoved in the special attachments folder. Things in there are 'public' in a sense - anyone can see them. So those attachments are viewable by students.
If the attachment came from the Resources (in the original site), the Assignment in the new site has attachments that ponit to a file in Resources from the original site. The instructor can see those since they are a member of both sites. Students can't see them since they aren't a member of the previous term site and so don't have permissions to view Resources there.
We could change the id of attachments that go to resources to make the id go to a resource in the new site.