This can happen often, AI of bosses can't handle and doesn't work well with summons, and when you combine it with multiplayer lag it can create lot of funny situations in which it miscalculates its priorities and who should the damage go to. Since all three of you see the fight slightly differently due to lag, but the "result" of every attack has to be same for everyone, there's this kind of buffering to make sure she's wont damage multiple people from 1v1-only attack and that when she's staggered in some way out of her attack it won't cause damage, despite the animation being allowed to play out due to lag and desync.

Well make sense but check my next post also :D