For a while, Solrock and Lunatone have been region-locked in two different parts of the world in Pokémon GO. Lunatone has primarily been in the eastern part of the world with Asia, Australia, and Africa, while Solrock has primarily been in Europe, and North & South America. For the summer solstice, Niantic decided to do a little switcharoo on the two and flipped regions on both characters.
In honor of the summer solstice, the Rock- and Psychic-type Pokémon Solrock and Lunatone have started to appear in different regions! Celebrate the longest day of the year by adding one to your Pokédex today!
— Pokémon GO (@PokemonGoApp) June 21, 2018
There's really not much else to this beyond the idea that for the next week or so, you'll be able to catch a creature you wouldn't have been able to get without traveling to another part of the world with Pokémon GO on your phone. The company didn't really give a timeframe as to when this would end, so we're guessing at best you have until the end of June to locate the creature you need and snag them before they get flipped back to what they were. It might be an issue now that the rise in players has gone back up over the past week. We wish you luck!