@lrnlab, @dklinger
Here is answer from Docebo Support:
There is a configuration in Advanced Settings, which I have active, that is called "Show the internal general catalog to the users not assigned to any specific catalog." This essentially means that if a user has not been assigned to any course catalog, then the user can see all the courses available in the catalogs that were assigned visibility of internal. Once the user has been assigned a course catalog, then they will only have visibility of the courses in their assigned catalogs, which I believe is the behavior you have been experiencing. I see that none of your users have been assigned to course catalogs, therefore they can all see the courses that are in the platform at this time.
In this case, if you deactivate the option "Show the internal general catalog to the users not assigned to any specific catalog" then your users will no longer see all the courses available unless they are assigned to a course catalog.
It is by design by using the configuration. Hope that it will help