Chimera Invulnerable

I just died 6 times on Chimera but it wouldn't come out phase its phase. I don't know if it's a bug but it sure seemed like one. Chimera was using that attack with the single hit dogs that spawn on you. Chimera wouldn't appear but just used that ability until i would die. Again, no actual boss was in the arena except maven buff. Chimera just disappeared and would not reappear.

Do you mean when he spawns the 6-7 clouds that you have to walk through to find him and you constantly get hit until you do?
If he is in the arena he is vulnerable. If it is when he is 'in the stands' and the adds come then you need to kill the adds to get him to come back.