Product Attribute filter not working

Hi,

I’m having trouble with the product filter AND the fieldmapping of g:brand. Due to our product data source, I’ve set the brand in a product attribute, called pa_merk. In the whole site, I use this multiple times, and works great. In the plugin, I can’t get it to work. When I put it in the filter, it won’t show results and the g:brand field is never mapped.

I am pretty sure there are products with this value, because it works in a shortcode context. Is there something I’m missing?