Rathbreaker bug.

I was fighting Rathbreaker after just getting to act 2 and died a few times, after dying I tried pulling him down off that platform, only to realize when I go back up to fight him as he doesn't follow he also doesn't do any of his special attacks or call the wolves up to him.

I ran into this as well. You have to fight him on that small piece at the top where he is, otherwise if you try to lure him down he will run back up and become invincible. The only way I was able to reset the invincibility bug was to die. After that I could fight him normally again. Hopefully they fix it.

for me he didn't become invincable, he just only did his melee attacks and never summoned or did his dash spear thing.