Why do FulfillmentOrders disappear in order processing?

I have an Order that has 2 line items.

Both items require_shipping and product_exists.

Last time my app saw the Order, there were 2 FulfillmentOrders. One for each line item. (They were assigned to different Locations).

Now when my app sees the Order, there is only 1 FulfillmentOrder.

(Aside: There are 0 Fulfillments.)

What happened? How does a FulfillmentOrder get deleted?

(No, the non-deleted FulfillmentOrder does not have 2 LineItems)

A bit hint that I’m seeing is that the LineItem of the deleted FulfillmentOrder has a 3rd party fulfillment service assigned, whereas the LineItem of the non-deleted FulfillmentOrder has “manual” assigned as the Fulfillment service. But this is still weird to me because there is no API to delete a FulfillmentOrder. Is it a permission thing? Is the 3rd party Fulfillment service the only app that can see the FulfillmentOrder? But my app did see the FulfillmentOrder before. Could the permissions be a new change?