Catalog Search Facets API / Portal Search Result Filters do not return information for products unavailable and set to be shown without stock. The product will appear on the Search Result, but its filters will not appear.

• Create a category
• Create a field group, within that category.
• Create an SKU field of the category, containing two values (that can be used as filters).
• Create two products, with one SKU each. And assign a different SKU field to each SKU. (important: do not assign stock to the products and make them show up on the page even if they are out of stock)
• Go to the category page;
• Check the SKU filter not appearing - same response of the Facets API /api/catalog_system/pub/facets/search;

There is no workaround.