After cancelling fulfillment for a line item on Shopify, why fulfillable quantity does not recover?

Shopify Partner
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Hi All !

We have an app to manage Shopify order status .
We recognize the order can be processed or not according to the fulfillable quantity. 
But we have an issue about order fulfillment.

When a line item(with 1 quantity) is marked as fulfilled, the fulfillable quantity is 0 for this line item.
Then we cancel fulfillment of that line item, it changed to unfulfilled on Shopify. 
Fulfillment status is Canceled as well. 
But the fulfillable quantity does not recover to 1.
So we can not make the item available again to merchants. 

It would be great if we get can any help about this. 
Nice day.