When a credit note is created from a Purchase Order, the credit note invoice is not using the same exchange rate as the one from the purchase order invoice. The steps are:

1) Create purchase order and post receipt.
2) Invoice the purchase order and check the voucher transactions.
3) Go to the currency exchange rates and create new exchange rate.
4) Create credit note from the purchase order that use the new exchange rate.
5) Invoice the purchase order credit note and check the voucher transactions.
6) You will see when checking the purchase order and credit note transactions that it's using a different exchange rate.

There's a workaround of using the fixed exchange rate in the credit note but this requires more steps for the users and there's a high volume of transactions being entered in the system.

In other process like project credit note invoices, the invoice is using the exchange rate from the project invoice but this scenario is not doing the same for the purchase order credit notes.