Vastweld, The Colossal Guardian unable to hit.

Was doing the forge map and when I got to the the boss fight started off normally but immediately after he used the attack where he changes his position I could no longer hit him with any of my attacks. I'm running a chaos damage over time build. Could not hit him with essence drain, contagion, or my curse. He was however being affected by withering presence. I stayed in the fight waiting for him to change his position hopping that it would fix thing but i spend a good 10 minutes and he didn't use that attack so I had to Alt F4.