Declined CC behaves as if approved

I attempted a CC payment this morning using the "Attempt Payment" button.

The payment was declined, yet the customer still received a payment confirmation email (with a copy of the invoice marked as "Paid" attached) and their hosting due dates were rolled forward to the next month.

So even though WHMCS left the invoice set as unpaid and sent an additional "Payment Declined" email to the customer, in every other way it acted as if the payment was approved.

I had to manually roll all of the due dates back a month (the invoice included 10 products) and explain to the customer that the payment confirmation was an error.

Is this normal behaviour for WHMCS or have I set up something incorrectly?

It says "Declined" with the following details ...

Action => Capture

Client => xxxxxx xxxxxx

ewayTrxnStatus => False

ewayTrxnNumber => 12345678

ewayTrxnReference => 1234

ewayAuthCode => 000000

ewayReturnAmount => 0

ewayTrxnError => 91,Card Issuer Unavailable

The strange thing is that I attempted the payment again and it was declined again, but this time WHMCS didn't roll the due dates forward, although it did send a Payment Confirmation email with a "Paid" copy of the invoice attached.