This is one of the new missions including in the Pokemon Legends Arceus update “Daybreak” that the recent Pokemon Direct foretold.
To find this one you’re going to want to head to the Heights Camp in Obsidian Fieldlands. There you’ll find Mai and her Munchlax hanging out and looking for you.
She tells you that her Munchlax has been having this strange issue lately where it just up and runs off. She asks you to look into it but she has no idea where Munchlax runs off to. Luckily Arceus loves meddling and hooks you up with a destination on your phone. It shows that Munchlax has been running off to the Ramanas Island.
Now, listen. I know that you know that Munchlax doesn’t swim so there is no way that this makes any sense but we’re just gonna ignore that. Right?
Anyways, you head over to the spot on the back of the island where, surprisingly, you find an outbreak of Hisuian Zoruas. It’s especially odd as these Pokemon never hang out in this next of the woods. You can either head back and report your discovery to Mai or, do what I did, and capture them all for the sake of expanding your Pokedex entry.
Once there Mai speaks about how she also thinks it’s odd that Zorua would be in this area. That’s when you notice that there are two Munchlax.
After a little bit of shenanigans, you head back to speak with Cyllene who posits a guess that the space-time rifts are causing bizarre outbreaks throughout the region.
With that, the mission ends and you’re gifted with 30 Ultra Balls, 60 Apricorns, and 60 Tumblestone.
This also leads you directly into the next mission which is entitled “Massive Mass Outbreak in the Fieldlands”. See you over there.