they have used for the past three-four years, a a trans-fat reduced formula
Sorry, you are mistaken. They promised to but never did.
Well, am closer to right than you may think; and as you are close as well. . .
They did reduce their 'trans-fats' and they did stop using the beef tallow; but they do still use 'beef flavorings/extracts' in their fry formula as well as in other consumables. . .(buns; bisquits(?)) at least here in the U.S.A.
And while I had a 'cheeseburger' in Ireland, a few weeks back; it did not taste quite the same as my local 'cheesburger' experience. . .and perhaps that is the reason. . .or maybe the density of the rain, blowing sideways by howling winds into the establishment, by way of every new hungry arrival. . .just made it all taste different. ;^) /don't know!. . .
Still not sure, who gets the cigar here; if one at all. . .