That is correct. The search will only filter based on the default products. If you wanted to remove this restriction, on the Product Results page you will need to modify the query.
Please look at line 347 of Product_Results.php and remove this from the end of the query:
WHERE COALESCE((SELECT GroupingID FROM productoptions WHERE productoptions.ProductID = products.ProductID LIMIT 1), ProductID) = ProductID
Note: When you remove these restriction, then all the individual product option combination will show on your results page, and depending on how many products you have, this could be a long list.
