In many countries, flour and processed grain products are required to be iron and/or niacin enriched, and many times the niacin used is derived from pork. This renders the grain product non-kosher. But why is the pork-derived additive not bittul when it is almost certainly less than 1/60 of the final product?
I think you are referring to the question whether bittul b'shishim is valid when done on purpose (lechathila). Here is what the Star-K writes on this
One may not mix even a small amount of non-kosher food into kosher food. This is known as “ain mevatlin issur l’chatchila.” Similarly, one may not put a small amount of milk into a very large pot of meat, even though the milk will be batel b’shishim. If one did this intentionally, he, his family, and the person for whom he is mevatel it may not eat it. However, if a gentile company adds a non-kosher ingredient and the non-kosher ingredient is batel, a kosher consumer may buy this product as there is no prohibition of “ain mevatlin” for the gentile. This is only true if it was not done explicitly for Yidden.
As you see the specific circumstances matter a lot which is why it is hard to rule in general.