Nintendo and PlatinumGames teamed up to bring Astral Chain to the Switch, giving us yet another example of the two companies working together. That's why it's a little odd to see Platinum update their official page for the game by removing mention of themselves.
If you visit PlatinumGames' official hub page for Astral Chain on their Japanese website, you'll notice that PlatinumGames has been removed from the copyright section. Just Nintendo is listed in that area now, and we have no explanation as to why.
For what it's worth, the English version of this website still includes both Nintendo and PlatinumGames in the copyright section. That leaves some wiggle room for this being a mistake, but we'll keep an eye on it.