Give the collection's products a tag and set data-tag on the block, so the search and its filters cover only those products.
The symptom. On a large collection, a search misses products, or the filter counts include products from outside the collection.
Why. Shopify has no search inside a collection: Smootify crosses the search results (up to 1,000) with the collection’s products (up to 2,000), and the filters count the whole catalog.
The fix. Tag the collection’s products in Shopify, then set data-tag="<tag>" on smootify-search-discovery. A search then looks only at products with that tag, and the filters count them. It needs the Tag filter turned on in Search & Discovery.