Were the miles used to make the booking earned by flying, or by transfers from, e.g., a credit card loyalty scheme?

Cancelling an award flight is not a mileage-extending activity. If the miles used to book the award would have expired by the time of the cancellation, then there simply are no miles left over to refund to you.

When did you last earn XPs in your Flying Blue account? If the answer is "never", or more than 2 years prior to the date when you cancelled the award ticket then there would be no miles to refund when you cancelled the flight.