I wonder if it could be one of the ingredients that just isn’t compatable with some people’s biology.
I’m reminded of Haribo Sugar Free Gummy Bears (aka. “Devil Bears”). For anyone who doesn’t know what I’m talking about see these links:
(EDIT: The forum software makes the below Amazon link look like an add, but it is the link I intended, to the reviews page for the Devil Bears)
They are an internet-famous candy that causes, uhm… comedically horrible gastointestinal distress… in a sizable percentage of the people by whom they are eaten. Some people are completely unbothered by them, but the about 10% or so of people that are affected have a very bad time.
I believe that for the Devil Bears, the culprit is Lycasin, a sugar substitute used in the candy. I wonder if perhaps the Soylent food bars have a similar thing going on, just with a different chemical and effect.
Also, another thought: possibly the batches the bars where made from were not mixed evenly or something, such that some bars have a higher concentration of the mismixed chemical than others. That could explain why some bars from the same batch are fine and others are bad. A low probability, but still worth consideration at this point.