Our warehouse who logs in with API credentials is getting the 422 error. They can access orders, but are not able to then send back tracking information.
I was able to find this in another thread, "The inventory_management field on the variant had been reset to "shopify". I set it back to our app's name, saved the variant and was able to create the fulfillment." I have no idea what it means or where to access this information/settings.