Explanation needed. Great Mothers vs KB

I think it's a bug with this part of his unique kicking in. 

With having his protection disabled, the game thinks he still has protection thus he can't drop below 100% health (100% of 1 is 1) so he became immortal.

I've noticed if you use a team against GM, you get credit for full protection with it disabled. 

You should Doomed someone else and focused him so you can do enough damage to take out his protection in the mean time.