Refunding an order response: "Arbitrary refund not allowed"

Start by reproducing the failure against the orders/{order_id}/refunds/calculate and orders/{order_id}/refunds endpoints using a real payment, rather than test mode. Compare the calculated refund and submitted transaction amounts with the issue's payload and inspect the shopify_api 21.4.1 refund implementation. Done means the cause is identified and a verified fix or documented upstream limitation explains whether the adjustment and refund can be created.

Issue summary

We are refunding an order but the item being refunded has return shipping cost (a variable we use on our side), so we are subtracting that from the amount.
This is failing on a productive shop, but if we use test mode or bogus payment method the error wont appear and the refund will be created.
In the tests environments the refund is created with an order_adjustment. In the other hand when we try refunding in productive shops neither the refund nor the order_adjustment are created.

EXAMPLE:

return_shipping_cost_cents = 1 USD
maximum_refundable = 20 USD
amount = 19 USD

• shopify_api version: 21.4.1
• Ruby version: 3.0.6p216
• Operating system: Debian GNU/Linux 11 (bullseye)

Expected behavior

Shopify should create the adjustment to the order and refund the amount.

Actual behavior

The error message: {"errors"=>{"base"=>["Arbitrary refund not allowed"]}}

Steps to reproduce the problem

1. Get order_id and line_item_id from an order with a real payment (not a test mode payment)
2. Call to the /orders/#{order_id}/refunds/calculate endpoint (POST)
   Sample payload:

{
"refund_line_items"=>[
  {
    "line_item_id"=>"11922901111117",
    "quantity"=>1,
    "restock_type"=>"no_restock"
  }
],
"shipping"=>{
  "full_refund"=>false
}
}

3. Create a payload to be refunded with the amount (amount - return_shipping_cost_cents)
   Sample payload refunds endpoint:

return_shipping_cost_cents = 6,95
{
  "shipping": {
    "amount": "0.00",
    "tax": "0.00",
    "maximum_refundable": "0.00"
  },
  "duties": [],
  "total_duties_set": {
    "shop_money": {
      "amount": "0.00",
      "currency_code": "USD"
    },
    "presentment_money": {
      "amount": "0.00",
      "currency_code": "USD"
    }
  },
  "return": null,
  "refund_line_items":
  [
    {
      "quantity": 1,
      "line_item_id": 11922901111117,
      "location_id": null,
      "restock_type": "no_restock",
      "price": "62.97",
      "subtotal": "62.97",
      "total_tax": "4.25",
      "discounted_price": "62.97",
      "discounted_total_price": "62.97",
      "total_cart_discount_amount": "0.00"
    }
  ],
  "transactions": 
  [
    {
      "order_id": 4786111111837,
      "kind": "refund",
      "gateway": "shopify_payments",
      "parent_id": 6017111111269,
      "amount": 60.27,
      "currency": "USD",
      "maximum_refundable": "67.22"
    }
  ],
  "currency": "USD",
  "note": "33HxxxYvLq"
}

4. Call orders/#{order_id}/refunds:
5. Response: => {"errors"=>{"base"=>["Arbitrary refund not allowed"]}}