The content is certainly there, and is recognized in this location by the Library Manager. Also, you can see that “All Media Types” is selected.
I was able to access the content in question yesterday through the Media Bay, and now it is blank.
Resetting the filter does not change anything. The folders still appear blank inside when double-clicked. The only thing that fixes their appearance in the Media Bay is quitting, then restarting Cubase and flipping a coin. Sometimes they appear. Sometimes they do not.