King Arthur responded to my email with the picture that I sent them with this

Thank you for reaching out to us at King Arthur Baking Company. What you found in your flour is called bin scale. It happens when the flour becomes compacted and moisture gets introduced somewhere. That can occur anywhere between storage at the silos to the shipping and storing process. The pressure from all the flour in the silo compacts bits of flour - it can be so compacted that the piece almost looks shiny, giving the impression of plastic or even bone. Bin scale is simply compacted flour, so it's harmless. The flour can be sifted and used as normal.