I just tried it as I suggested, and it does indeed work like that… I selected the “product_cat” Mix N Match as in my example and my new test field, only shows in my products that belong to that specific category.
Rather than selecting each individual subcategory of it, tho, I editted my products to be included in that parent category as well.
Have you tried adding an “AND” filter to the field group with a “Post Taxonomy”?
For instance: http://prntscr.com/o1vm5w