Attribute filter not working for one attribute, but works for others

The filter for the brand attribute is not working, although other attributes are. When I click the brand filter it shows no products. However, if I change the URL from wpf_filter_brand to just filter_brand it does work.

Of course that causes issues when I try to combine filters.

So how can I get it working again with wpf_filter_brand? It used to work…not sure what could have changed.