When a sales quote, sales order, or sales invoice with a discount is copied to a purchase order or purchase invoice, the discount does not show on the resulting entry form. But the completed document, when created, has the discount from the source sales document.
You might debate whether a discount from a sales document should ever be carried forward to a purchase order or purchase invoice, but if it is, the entry screen should definitely show it.
In both the purchase order and purchase invoice cases, if the
Discount box is checked, no discount shows on the entry form, yet the discounts appear on the completed documents. If the box is checked and then unchecked before the transaction is created, the discount is cleared and does not show on the completed document.