Every now and then, Nintendo will release a day-one update for a new game that is about to launch. The update usually releases a few hours before midnight at launch day on the east coast of the US. However, there are some occasions when Nintendo will release the update even sooner than that.
This is one of them. Nintendo has released an update for Pokemon Legends: Arceus, even though it is still almost 2 days before the game releases. Nevertheless, once you have the game and update it, it should be at version 1.0.1. As for the patch notes, all Nintendo says is that they “fixed some problems for a better gaming experience”.