The Caged Brute

I have killed Brutus twice now and am not receiving credit for the kill. I cannot progress without it flagging the kill happened. First time I had to go all the way back to lower prison create a new instance for upper fight back to warden's chambers then kill him again. Is this just going to keep happening? What do I do?

Just went back and killed him a third time. Still no credit. The first kill I was using caustic arrow and ice shot. I thought perhaps he died to caustic so ditched that for the second kill- only used ice shot on second kill. Third kill I killed him using burning arrow. Good stuff. smh