Hello, id like to know if anyone ever got this problem with the effect damageResist.
Also, did you manage to correct this problem?

Im giving 55 DR to super mutant master and the end up with the max (99 in my game) meaning they are almost invincible
tryed with 40 DR and it take 3 hit of 500(missile launcher) to kill them (they have 375hp)

Im trying a few thing to compensate for now to remove this problem

It's intentional, AFAIK. Presumably as they can't wear armour so they should gain double the effect from any DR-boosting drugs etc. to make up for it.