Unexpected fulfillable_quantity: 0

Shopify Partner
54 0 11

I host an app which creates shipping labels, it does so + fulfill in a single operation. 

One of my customers since (at least) yesterday seeing some orders with "fulfillable_quantity: 0" on all items, despite there being inventory left. This is very much interfering with their workflow, and making the app look disfunctional in front of the customer.

I have access to the customer's admin, but I'm currently not able to understand what seperates  orders with zero fulfillable_quantity from orders with "normal" fulfillable_quantity. I can't pinpoint anything in the GUI or in the JSON.

I understand that shopify is doing some changes on fulfillment status, so that could be related. 

If anyone else is experiencing the same problem, and/or do know what can cause this issue, please let me know (Shopify support I'm looking at you).