The refinementList widget will filter the data set based on the selected value(s) .
Optional filtering returns records that match both the query and the filters, but it also returns records that do not match the filters. The effect is on the ranking: records matching the filters are ranked higher than records that do not match the filters.
Refinement lists do not apply optionalFilters without customization.
Though this type of customization is out-of-scope for the Support team, you could achieve this functionality by creating a custom widget based on the refinementList widget which sets the optionalFilter API parameter rather than the filter parameter.