soul core of foulness not adding poison in toolbar

I have added three soul core of foulness (flat chaos damage) to my bow and have not seen any additional chaos damage added to my poison damage. I could be mistaken, but I thought that a bow with flat chaos damage increases the amount of poison on "Main hand estimated poison damage per second". That seems to only go down when the soul core chaos damage is added to the bow flat physical damage. Please correct me, if I am mistaken. Thanks.