Incorrect "subtotal_amount" on Fulfillment event when discount is applied

[Avalara]

Hello Shopify Team,
    We are seeing an issue where in one scenario subtotal_amount under cart_lines -> cost is inaccurate when we receive event on order fulfillment.
Here are the steps to repro the issue:

1. Create an order with a product by adding 5 quantities of it. Say each item is $20 and therefore total amount is $100 pre-tax.
2. Estimated tax calculated on the order is $10.30. So Total Amount of the order is $110.30
3. Now add discount to it using the following steps:
   1. Click on add discount -> Add custom order discount - > Discount Type as Percentage  and add 10% as discount value
   2. Apply discount to order
4. This discount reduces the total amount to $90 and hence taxes are recalculated. Taxes now comes as $9.27 on discounted price
5. Total amount post estimated taxes and after discount is now $99.27
6. Click on Collect Payment  -> Mark as Paid
7. Now we will fulfill the order in 2 batches.
8. Click on FulFill Items button and change quantity to 3 (of total 5). Click on Fulfill Items.
9. This partially fulfilled the order with 3 quantities. But the event we received has the following data:
   

                     "cost": {
                       "amount_per_quantity": {
                         "amount": "18.0",
                         "currency_code": "USD"
                       },
                       "subtotal_amount": {
                         "amount": "50.0",
                         "currency_code": "USD"
                       },
                       "total_amount": {
                         "amount": "59.58",
                         "currency_code": "USD"
                       }
                     } 

   Please check subtotal_amount above. That seems incorrect.Now let's fulfill the remaining 2 items as well. For that the event received has following data: (Please see subtotal_amount )
   

     "cost": {
                       "amount_per_quantity": {
                         "amount": "18.0",
                         "currency_code": "USD"
                       },
                       "subtotal_amount": {
                         "amount": "30.0",
                         "currency_code": "USD"
                       },
                       "total_amount": {
                         "amount": "39.72",
                         "currency_code": "USD"
                       }
                     }

   Due to this, we (Avalara) are seeing discrepancy on order amount in Shopify and Avalara Avatax during transaction recording. To me it looks like it applied full discount of $10 on subtotal_amount on each event.

Please suggest!