I know Americans who have the same diet in the US and Europe and they always lose weight as soon as they come to Europe
What I have noticed is that buying from my local farmers market has helped a ton in the U.S. I still get some staples from the supermarket, but stuff like meat only comes from the farmers market now, and even if it might be significantly more expensive, it’s absolutely worth it because my health is doing so much better (even if I couldn’t objectively link it to this change, I wouldn’t be surprised that eating chicken that was raised running in an open farm and eating wild food, that I have seen with my own eyes is better for me than chicken living in their and thousands of other chicken’s feces).
Even just between US cities, there are different foods at the grocery store at different price points that would make it unreasonable to stick to an identical diet, so it's hard for me to believe that everyone in these anecdotes has identical intercontinental diets.
Yet these anecdotes are never about obvious differences that could explain large calories swings. They always focus on cocksure yet vague claims about chemicals and additives.
It reminds me of people claiming that switching from corn syrup to cane sugar Coca-Cola, a magical elixir, when they traveled to Mexico must obviously be the responsible for some bodily change they allegedly experienced with no further interest in any competing explanation.
Also, sugar and fructose increases appetite levels.
HFCS (high-fructose corn syrup or isomerised sugar) is corn starch which is turned into glucose/dextrose by enzymes, after which about half is further turned into fructose giving it a sweetness like sucrose (table sugar) or honey so it can be used as a replacement for the already cheap sucrose or more expensive honey. Consumption of glucose containing sweeteners has no benefit for a typical sedentary person but does mess with your blood sugar because it goes directly into your blood once free.
Fructose, also known as fruit sugar because the sugars in fruits are mostly fructose, does not spike your blood sugar because it first has to be converted to glucose by your liver. Glucose is only half as sweet as fructose for humans, but since sucrose (table sugar) and HFCS are a little cheaper to produce and get consumed at a massive scale, in practice a consumer will have to pay at least 3 times as much for pure fructose.
For the typical HN user without medical issues and the disposable income you could aim for no sweeteners > sugar substitutes > fructose.