I messaged Soylent directly two days ago (the same time I first posted my problem to this tread). They just emailed me back a few minutes ago. They refunded my order, and also are providing me with a shipping label so that I can send the remaining suspect bars back to them for investigation.
So they are definitely taking this problem seriously and, it seems, doing their due diligence to get to the bottom of the issue.
This sort of great customer service is just one of the reasons that I will still be a customer in the future (once this issue is fixed), even after a mishap like this.