It was from the whey powder that they use. I am a bit surprised that it happened, but I appreciate their honesty.
Soylent has a pretty bad record. First bars that made you throw up, then mislabelled drink, now this.
I thought they handled this as well as it could be handled. Clearly a problem in the co-packer facility, with the problem isolated to one batch; they emailed all affected customers and offered replacement shipments; they promptly notified the FDA; they put out a press release.
Well… I thought they were being prompt and proactive! Now, per a post on reddit it appears that nobody has received a replacement, or even confirmation that they will receive a replacement.
That’s very bad 
I’ve been part of the unfortunate batch of Soylent Bars, got my refund, but it took them a long while to actually process it.