Zalmarath not taking any damage after the walking bouncy laser attack

I am not sure what caused it I am using a freeze build and hit him with a curse and frost bomb while he was walking. After the curse and frost bomb he stops taking damage when he is still. When he does the attack where he walks is the only time he takes damage. I got this to happen twice now.

got it to happen a third time now. I think it is something with the frost bomb, curse, ice wall, freezing him while he is moving or all four. When he does the walk attack again, if I do not press all my buttons he does take damage again when he stops.