You're too far away from her which makes her reset. My guess is you're trying to catch the circle in the air that she leaves behind just before she drops down. Had it happen as well, just stay closer to her.
It's a matter of distance and has been there for a while now.
It only has happened to me when the boss has been finishing the first molting or starting the 2nd but I'm guessing it can happen on every molting. Just make sure you're right next to her.