Oooof. Add this to the troubled history of Platinum Games. Not happy to hear this news at all. I'm not really familiar with GranBlue, but I liked the gameplay footage, but I can't deny that I was mostly interested in the game because of Platinum Games. I hope Platinum's other projects (Bayonetta 3, the Square-Enix game, whatever else they have in the pipeline) will be succesful enough so that this won't break their necks.
I'm a bit surprised by the amount of people saying stuff like "nothing wrong here, Platinum finished their development and now CyGames is just polishing it" etc. Even though we don't know any details I don't think that's the case. They wouldn't have to announce anything then nor would they have to remove Platinum's logo? The latter would especially be baffling. Removing their logo just screams that they're trying to erase Platinum's history / involvement from the game, which I think wouldn't be the case if they would've just finished the work on the game they were contracted for. There has to be more to it and we recently saw similar happen to the FF7 remake as well as Metroid Prime 4, where we got some early rumors, that Nintendo is changing the developer (which they recently confirmed). I remember people also saying that it doesn't have to mean anything back when the rumors about Nintendo changing Metroid Prime 4's developer spread around.