In the event of a biofuels delivery in December of year X, could you advise what an ISM-Company should do if the Fuel Supplier does not receive the relevant data on emissions from the producer before the deadline of 31 January in year X+1?

Unfortunately, you need a valid proof of sustainability, meaning in this case completed with all the required information and fields. If there is missing information, the fuel cannot be submitted as a sustainable biofuel.