IIRC, trans fats were banned in the US. I don't think it works be legal for them to contain them anymore.
Jiffy corn muffin mix doesn't contain partially hydrogenated vegetable oils and contains zero grams of trans fats per serving (I have seen it claimed that products may have less than 0.5 grams per serving and round down to zero).
https://www.fda.gov/food/ingredientspackaginglabeling/foodad...
They do also have a page for a Jiffy Baking Mix (no corn, sounds similar to Bisquick) that did contain partially hydrogenated vegetable oils, enough for one gram trans fats per serving.
[0] https://www.fooducate.com/app#!page=product&id=43DBDE62-E10F...
[1] https://www.fooducate.com/app#!page=product&id=43BFE144-E10F...