[HELP] Not auto reflected pricebook rate to items when creating a sales order by api

[HELP] Not auto reflected pricebook rate to items when creating a sales order by api

Hi,

When I created a sales order by API with the pricebook_id parameter, the created sales order items were not reflected prices by the price book.

The requested URL is like https://books.zoho.com/api/v3/salesorders?organization_id=XXXXX with POST.
JSON is like {"customer_id": "xxxx","date":"2021-05-27","shipment_date":"2021-05-27","line_items":[{"item_order":0,"item_id":"xxxxx","quantity": "1"}],"pricebook_id":"xxxx"}

As you can see, I don't pass the item rate. Because I think the item rate is determined by the pricebook_id parameter.
I received no error result and I can see the sales order in the zoho books dashboard (https://books.zoho.com/app#/salesorders).

But the item rate is set as the default rate in the sales order dashboard (https://books.zoho.com/app#/salesorders/xxxxx).

What I expected:
If one item in a pricebook is like below,
Default sales rate: 1000
Custom sales rate: 500
When I request to create a sales order by the above API, The created sales order item rate is set as custom sales rate, 500, automatically.

Question:
Is the behavior correct as of the specification or bug?

Thanks for your help