Complex PO is not able to apply against prepayment when no functional currency is used

Hello,
When a supplier submits a complex PO invoice through the supplier portal, the invoice should ideally be automatically applied to any prepayments. However, there is an issue where this automatic application does not occur if the invoice is submitted in a non-functional currency

In contrast, when the invoice is submitted in the functional currency the system correctly applies the invoice to prepayments. Additionally, if invoices are manually matched, regardless of the currency, the system processes them correctly

Any reason why this is happening?

Thanks.