This goes with my views on the Pure Illusion worlds of the first eight episodes. Pulled over from Twitter:

@cks_anime: Pure Illusion in Flip Flappers episode 9 isn't obviously based on anything (to me), but my theory is that we're seeing inside Yayaka's head.
It's certainly striking to me how the Pure Illusion world has a bunch of visual similarities to the locker room [that Yayaka is in] at the start of the episode.

[...]

@cks_anime: Also: Yayaka's desires clearly altered the world towards the end, which I don't think she's done any other time.
(It's not clear if she consciously willed things to happen or if the world just knew what she wanted & did it.)

While not all Pure Illusions worlds are tied into the cast, I think that the world from episode 9 is too on-point with what's going on with Yayaka and too visually similar to the real world scene we see her in at the start of the episode to not be related to her (and the world is the way it is even before she arrives in it).

I can also read the world responding to Yayaka's desires as similar to the Pure Illusion world in episode 1 responding to Cocona's feelings at the end of the episode. And it's now pretty clear that the episode 1 world really is Cocona herself.

(This all feels relatively obvious but I wanted to get it down anyways, and not just on Twitter.)