I've seen reddit thread about it. It was concluded that it was the addition of folic acid to flours in US. Most countries in Europe don't fortify flours or grains with anything.
A lot of additives that are allowed in US are banned in Europe. But it can be anything from wheat type to glyphosate to food fortification to fluoridated water to bromated flours to excitatory additives in food in general. Now they're introducing Mercosur, shutting down European agriculture and flooding markets with unregulated food produce from Ukraine, so EU food quality will go down as well. They want to ship food from South America to Europe. Some governments even pay farmers to shut down their businesses, it's wild and in US we've had farmland buyout. Food is not safe anymore, just an industry.