It could be that you changed something in the gear somewhere, or gained a new great hall bonus, that made your designated stun target slightly different, hence throwing off the stun target choice in certain specific circumstances. Don't think anything has changed with the CB. 

Maybe not just "random turns", remember he takes damage when placing unkillable. Nothing has changed in CB and neither of those champions have been rebalanced

What do you mean that he dies at random times?  Can you pinpoint more specifically when he is dying?  For example, is it when he casting unkillable, or when unkillable is off him and CB targets him?  It won't be random, and knowing the exact situation that is occuring when it happens will help identify what the cause is

Tbh something was realy up with R&G yesterday I was farming the same dungeon lvl all day then in the afternoon (GMT) I started getting one shotted by the same boss I had been beating all day 7/8 times in a row. also CB gave me the lowest possible score I ever get 4 out of 4 times and teams I normally beat in arena were smashing me. 

Its all back to normal now so maybe it was a bug with the new update.