Welp, my theory was totally off. Matsuri didn't change back. The kiss didn't do anything.
Believe me though, no one is happier to be wrong than me. I WAY prefer yuri in all contexts and that the author is capable of such a playful misdirect to throw pattern-seeking people like me off the scent of the story... well that's honestly really cool!
YURI IS LOVE YURI IS LIFE.