Explanation needed. Great Mothers vs KB

Wish I had an answer for you but I had the exact some thing happen vs New Bad Batch wrecker today vs. Bane. I was hitting for over a Mil dmg and he never died, never could even get him in to captive. Obviously he was stuck a 1 health somehow.

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.