Lessons referencing assignments are broken after upgrade

After upgrading from an earlier version of Sakai, Lessons that reference assignments appear as "Deleted" ** on a site's Lessons page.  To reproduce this problem, do the following:

-Run all necessary database conversion scripts

-Bring up Sakai 12 server

-Log in as admin and add/run Assignment Conversion job, as detailed here: https://confluence.sakaiproject.org/display/DOC/Sakai+12+upgrade+information

-View a site's Lessons page.  All assignments will appear as "Deleted".  

Note that the problem can be fixed by editing the Lessons item and re-selecting the appropriate assignment.