There's a few different ways to look at it.

In essence, it's Rose having a manic episode. She's stopped caring about the big picture, and went no contact with everybody in order to focus on her destructive vices. Kind of like what her Mom did around the house. When Rose meets up with John, she's not possessed or brainwashed or w/e. But she is kind of embarrassed to be seen like this.

Textually, Rose is getting info from two sides by the time Grimdark happens: The Horrorterrors and Scratch. They're opposed to each other for reasons that we don't know this far into the story. I posit that Grimdarkness is something that was bestowed upon Rose in an attempt to break her connection to Scratch. Not out of malevolence, but perfectly timed to appear as such.