User blog comment:Kogath/Monster Hunter: World Discussion/@comment-31029678-20190208002724/@comment-4370375-20190209002720

Technically the reason those can be treated as non-canon is because they're not acknowledged outside of the event itself.

That's really not possible for either Behemoth or Leshen however as both have lasting consequences after the Special Assignment ends. Cactuar can still be found in the Wildspire Waste after Moogle goes home and Nekkers presumably can presumably as well since you get a bounty for one and Behemoth becomes an optional quest after the Special Assignment.

Leshen is a little more complicated since for now only Geralt can fight it but since we know the player hunter will be able to fight Ancient Leshen and Geralt gives the hunter the ability to use Ignis after vanilla Leshen is brought down, it's possible that there'll be a way to fight vanilla Leshen as the Hunter.