I'm Lizzy, one of the Gurus on the support team at Shopify.
After taking a look at your question I think this page from our help center might explain what is happening with this order. If you check out the Example section it shows the rules and priorities of fulfillment.
In this case, it looks like this rule is the one you're seeing in action: "If required, assign a line item to a location that has enough inventory, but do not split line items between locations."
What that means is that a product (line item) will not be split between locations. An order can be split if different products are available in different locations, but if multiples of a product are ordered, that are over the stocked amount at any one online fulfilling location, the order will default to the top priority location regardless of stock amount.
I hope that helps to explain what you're seeing. Please let me know if you have any other questions.