Lost-Men Brute random chill/frozen immunity

When in the Bone Pit area sometimes randomly the Lost-Men Brutes will not be effected by chill or frozen, sometime it will just not apply chill build-up and sometime it will apply the chill build-up but when it gets to 100% it vanishes and does not freeze them

I just had this happen today, I thought I was losing my mind. Same exact issue. Some brutes freeze, but most seem to be completely immune.