Exchange Rate Variance in multiperiod invoices

We have an issue in multiperiod accounting invoices. if the entered currency is foreign currency, the exchange rate variance is not generating.

We have customised the accounting rules, but we do have conversion rate variance defined in journal entry rule set similar to the out of the box JERS.

If Anyone else has faced this issue/ using MPA invoices, kindly let me know how the accounting rule is handled for ERV.