Once you show a 100+ivs OT cubchoo ot to her, you obtain the cubchoo hat, but after that if you go to poke center without talking to her again immediately she disappears without the possibility of a battle and this weird interaction happens:
Video: https://vid.me/mBrR
Edit: just a note, i already tried to press space or interact with the place where the boss is supposed to be, so not only missing in terms of graphical appearance