Over the last year Avanti have added extra time and extra stops into this service
AIUI that is being undone from September.
LNER have managed to stick to the usual ticket release cycle,
Avanti have been hit more than most by planning issues though the Covid period. I’m not going to explain why here, but much of it is not of their doing. Meanwhile, LNER, alone, have a system that allows them to sell tickets ahead of times being confirmed. This system (or something like it) will be rolled out to other TOCs in due course.