Incorrect exchange rate variance calculation after enabling the USD Reporting Ledger on PL

One of my customer we have enabled the USD Reporting Ledger on AUD Primary Ledger and ran the SLA historical upgrade program with Accounted values in Primary Ledger with single exchange rate.

There are few existing open Non-USD POs (AUD & EUR) and when we are trying to match those po to ap invoice, the exchange rate getting calculated in USD Reporting Ledger with the difference of PO creation date rate amount - Invoice date rate amount.

Customer expectation is to calculate the ERV amount from SLA historical upgrade rate given for conversion date rate to AP invoice date. The calculation should happen from cutover date for all existing POs created before the cutover date..