I understand revenue protection. A3 wants to keep it random where you can use the voucher, even if the J-cabin is empty. But my guess is, that they just expected a good load on this flight, which didn't happen and they didn't bother to manually clear the voucher at a later stage. I think so, because they were blocking I-class with a clear pattern the days and weeks before my flight.
If this was really a case of random/clever blocking due to revenue protection, then fair play.