At first, I thought that too. But it seems that the boss bar never got to a multiple of 25... Ex. 25, 50, 75, 100. The symbiont did die though, but I believe that is from an Insta /kill. Because the either storm head also died by that, (meant to be unkillable, just stunnable) that means the mutants just somehow bypassed the fronts invincibility and damaged it with their own means. And the creeper instakill.
