I'm with you but it's a common enough sentiment that there's probably something to it, and I recall friends having issues over the years. For these people at best it's painful gas and at worst is [worse], so there is a large group of people who can't digest a particular common Taco Bell ingredient well.

I've seen speculation about undiagnosed lactose intolerance but frankly Taco Bell doesn't use that much cheese on their cheesiest items compared to say, a pizza, which is another very common food in the US and has way more cheese.

I wonder if it's undiagnosed gallbladder issues too.

I haven't had taco bell in a couple of decades, but I used to have issues with the beef shortly after eating there (so probably not bacteria) but no issues with chicken. I suspect my body was not used to the grease, and that taco bell had particularly greasy beef.