I was just putting together a K-class itinerary from the UK to Australia through North America using QF flights as much as possible, and found that flights from JFK-YYZ are open in most classes but closed in K-class on August 15 and 16, the only dates I checked.

This seems to be different from K0, which I have seen before, but I am not sure in what way.

Any thoughts as to why one class would be closed while others, both higher and lower price one would think, are still open? Or why it would be closed rather than K0?