In Confluence, contents (pages, courses, etc ..) are indexed in order to be listed and searched for.
If your published courses are no longer displayed in the course catalog, then it is most likely due to a corrupt state of your Confluence search index, that occurs when upgrading Confluence, importing new spaces, server crash etc ....
What to do?
Rebuild the search index and check again.
If the problem persists, we think that the best way to proceed is to reach out to Atlassian support to find out more.