If you set namespace restrictions in Islandora's Configuration (admin/islandora/configure), objects outside the declared namespaces will not show up when browsing the repository.
But if you do not also restrict the namespaces in the Solr settings (admin/islandora/search/islandora_solr/settings and click Query Defaults), restricted-namespace objects do appear in search results.
Or, if you have those limits configured already, adding a new namespace in the Islandora configuration will require you to also add it to the Solr filter list, or the new objects won't appear.
Islandora should add the disallowed namespaces from the Islandora Configuration settings to the list of behind-the-scenes Solr query filters automatically. The list of namespaces in the Solr Query Defaults section should only be used to apply further limitations to search; they shouldn't be entirely separate.