Ah, I think I understand. All works as you expect EXCEPT authors?
The problem is that the data for Authors comes from outside the post and postmeta tables (the tables that S&F uses for its cache) and is built independently of the cache, just as the form is loaded. This means that the list of Authors is not filtered by making selections in the other filters. This has always been the case in the plugin, but I am not sure if this behavior will be changed in the next major version (V3). It will require that we extend the plugin to include the users table.
The only way to get around this for now is to make an ‘author’ custom field using a plugin (like ACF) that stores that data in the postmeta table.