Changing PO header currency - exchange rate date

Hi experts,

We have created a PO on 02.09.2019 and with currency USD and on the line item the delivery date was 09.09.2019.
Now we have changed the delivery date to 30.07.2020 and the currency to ARS and the system is taking the currency from the original delivery date (09.09.2019) (TCURR table/ ob08 tcode).
Any thoughts on why is this happening?

TCURR for those dates:

Line item before changing currency on header:


Line item after changing currency on header:


4,08 * 55,82 = 227,75

Thanks, regards.