I am the sysadmin. The mbz file does contain the user data. I have verified by examining the main gradebook.xml file in addition to the individual activities/assign_xxx/grades.xml files. So, the backup at least appears to have completed successfully.
In fact if I do a backup of the restored course (where the gradebook appears empty), download the mbz, extract it, and examine the gradebook.xml and grades.xml files all of the data is there as well.
So why isn't the data visible when browsing the course?
Also, it isn't just that we need the data (as I have a very good backup strategy so recovery of the data is never in question) it's that we need to be able restore the course from the mbz backup file on a new installation. We need to be able to move completed courses from our production Moodle installation to our archive Moodle installation otherwise the production system becomes very cluttered with old courses.
Any other suggestions? I keep feeling like I'm just missing some setting or option. Being that all of the data is there on the archive server (I also verified it's existence in the db tables) there should be some way to make it appear in the web interface.