Srcset not working for woocommerce category images

Does anyone have an idea why srcset may not be initialising for all thumbnail images on a site?

On the site I’m working on, catalog, single product images and other non-shop content is fine. E.g.:

http://www.curioustea.com/tea/black-tea/

But product category images do not have srcset:

http://www.curioustea.com/tea/

I’ve tried turning off all plugins and changing the theme but no difference.
Also, if I change the shop home page to display categories and products, the product thumbnails have srcset but the categories still don’t.