Facets are essentially counts of all unique values in given result set.
So if you apply a filter on a particular field and facet on it, it won’t include the other non-filtered values, since it’s not part of the result set.
This is the reason for eg, in the ecommerce demo, the first search in the multi-search request does the query with the filter applied, then it does another query without the filter but with just the facet, to get facet counts for all values