[Cannot change quantity]

Hi everyone,

I am creating an integration for a niche ERP that will be used by multiple organisations pushing their invoices to their Sage account.

The issue I am struggling with is setting invoice line item quantity; it just won't work. Besides that the discount is also not working, which I assume based on API responses that my demo account is not on the appropriate tier to have discounts. Anyhow, back to the quantity issue:

My body looks like that

{
	"sales_invoice" : 
	{
		"contact_id" : "bcd33c2ba23e4345878e3ce3e6a62da2",
		"date" : "2023-01-12",
		"invoice_lines" : 
		[
			{
				"description" : "Test description",
				"ledger_account_id" : "cc494e26929d11ed84fa0252b90cda0d",
				"net_amount" : 675,
				"quantity" : 3,
				"tax_rate_id" : "GB_ZERO",
				"unit_price" : 225
			}
		],
		"reference" : "INV-473E270D09"
	}
}

The result of this update call (PUT to /sales_invoice/{{id}}) is attached. Notice that I cannot enter quantity in an invoice not even using the Sage normal front-end.

May I assume that quantity is again another field conditionally available based on subscription tier ?