FYI, the TN operated flights are airshare agreements, not codeshare ones. From my understanding, *G benefits don’t apply (but will on the NZ domestic sector).

The difference between airshare and codeshare is that with codeshare, the marketing carrier can theoretically book every seat on the aircraft under its flight number. In contrast, airshare means that the marketing carrier has pre-purchased a block of seats on the operating flight and can sell them how it likes. This is probably why you are seeing availability change constantly as there are people booking/cancelling all the time with a very limited pool of seats.