I tried to filter for the brand but never got any results back. Brand was a localized text attribute on my products. I used the following variations:
variants.attributes.brand:"LEGO"
variants.attributes.brand.de:"LEGO"
masterVariant.attributes.brand:"LEGO"
masterVariant.attributes.brand.de:"LEGO"
I'm using the node.js-SDK btw.
the pattern for filters on attributes of localized text type is
variants.attributes.{name}.{lang}:"{value}"
To analyze why this is not working for you, please create a support ticket on https://support.commercetools.com
Thanks Stefan