US kitkats are a Hershey product, internationally they're a Nestle product. And of course Hershey is one of the worst chocolate brands on the planet as far as taste goes.
Long story short, Hershey chocolate has butyric acid in it, which tastes like vomit. Hershey doesn't add butyric acid to their chocolate, but it does come from milk going rancid(it's in other stuff too, various cheeses and things), so the general assumption is that Hershey does something to the milk they're adding to their chocolate, likely to aid in shelf stability(because butyric acid does that) but won't admit to(they don't have to) what their actual production process involves. because Hershey won't admit to how they process their milk or chocolate in general, there's all sorts of rumors/conspiracies/whatever about this, especially since most other decent chocolate brands don't have this issue. And that's before getting into the weird part about Hershey chocolate having a rubbery consistency at room temperature(also disgusting, and noticeable with a plain Hershey bar), so who knows what causes that since it isn't normal either. This would be why europeans tend to complain about american chocolate, because theirs doesn't have this shit in it.