We have a supplier whose default terms is set to 14 days rather than the default of 30 days.
When we make a copy of an existing purchase for this supplier, the supplier’s terms are ignored and the payment due date is set to 30 from the receipt date.
It works OK if the supplier on the purchase is modified and reselected, but has a side effect of clearing the VAT rates. It also works OK if a new purchase is created for that supplier. Sales invoice terms seem to work correctly when copied, so it just an issue on purchases that are copied.