It looks like Bricks filters only gets the attribute slug instead of the field name slug. Another taxonomies such as Brands (San Pellegrino, Watter matters,…) prints the field name instead of the slug as expected.
Is there anything that I’m doing wrong or is this a little bug?
Sure, but doesn’t work with my custom grid made of variation products instead of variable or single products.
First filter (working) Source: Custom field Meta key: attribute_pa_sabor
This works well for variations because you’re working directly with the postmeta of the variations, and variations have those values as custom fields (attribute_pa_xxx), which allows you to filter variations directly.
Second filter (failing) Source: Taxonomy Taxonomy: pa_volumen (Product volume)
Issue: Variations are not registered as terms of the pa_volumen taxonomy. Even though the attribute is visually assigned from the admin, WooCommerce does not store the taxonomy relationships in wp_term_relationships for variations — it only does so for the parent product.
Therefore, when you use a taxonomy-based filter, you’re querying for products that have that term relationship. But your variations don’t have it, even if they visually appear to use that attribute.