I have a number of clients with this same issue. When running CNP transactions, they often have the CVV, but sometimes do not. They set MX to allow for mismatched CVV, but CVV still remains a required field. They put in a "dummy" CVV (IE 111) and then the issuer declines due to CVV mismatch. Often the same card can be ran with CVV omitted completely and will process normally. But again, MX is forcing a CVV to be entered.
While the merchant has the option CVV off completely to run the transaction, disabling it in the settings, running the transaction, and then re-enabling CVV each time is not practical.
Therefore there should be an additional setting to prompt for CVV but make it an optional field.