Zalmarath, The Colossus goes full imune to everything when he walks to right side

It Doesn't happen every time but like 4 out of 5 times. this is in Act 2. It is the first time happening to me and i beat this boss as a monk (dark one), sorceress(chronomancer) and witch(fire one) with out it happening. But now it is happening when I am trying to beat it with mercenary (gemling).

Also the rolling of the platform is an annoyance but not as bad as the immunity.

Bug report reference number is 862,164,792