Whenever I pushed her to the next phase with a big Erasing Strike swing (Aka emptying her health bar/s) , the skills corpse removal turned the boss invisible and I could not see any of the phase transition animations, only the sound was heard. (Pretty crunchy ones at that. 
)
Whatever she is doing there, it’s pretty bloody. Can’t see it though.
This is only for said animations, when the fight resumes she is perfectly visible again. HOWEVER, I assume there were changes to her model for each phase? If that’s the case, the bug prevented those and she looked her normal self like she does in the first phase.
Hope this helps.