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 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.
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.