I saw that too, and it is odd. But “dextrin” isn’t lactose. And this is the manufacturers website, so if they explicitly say it doesn’t have lactose then that’s pretty much from the horses mouth. I didn’t immediately see a nutrition label.
The “side effects” are the same things you’d see on pretty much any product… eg, placebo’s in drug trials usually have the same side effects. I’m sure plenty have reactions like this to certain gels, which is why there’s so much variation in what people prefer and tolerate.