Adding a Miscellaneous Charge to an AR Invoice changes the Line Value

We are adding Miscellaneous Charges to our Invoice Lines for an Energy Surcharge. This works fine for base currency, but for other currencies when I add the Miscellaneous charge, often the base currency Extended Price value for the Invoice Line changes (I assume is re-calculated) and gives an incorrect value for the line. (A few Pence difference)  This then means the Invoice won’t post because the debits and credits don’t balance.

Has anyone else seen this with Epicor? We are on 10.1.400.