I'm not sure about the Wonder but on Oasis and Allure if you want to book Izumi you need to book it in person at the restaurant (unless you're prepaying the meal ahead of time for a certain time), it's the only restaurant on board to have this requirement because it books so fast and seating is very limited. As RCCL has a 48 hour cancellation policy many people will book the prepaid slot ahead of time for later in their cruise week and then cancel once on board to replace the booking with their Unlimited Dining package, the refund is usually then done as on board credit. There is no guarantee though that they won't change this policy later and refuse the refund once on board.