The resistance displays their actual current value (it's tallied as you open the window) so it will exclude it if they've lost the relevant part, or if it's inactive or disabled for some reason. Behemoths show all their proper resistances and always have for years, so I'm guessing there was probably some other factor at play you overlooked.
Oh actually, now that you've mentioned the Behemoth specifically I think I know what you're referring to. Recall that many bots also have
negative innate resistances, but in the case of some of them they have other parts which
simultaneously offset those negative resistances with an equal positive one via utilities! If you destroy those parts they suddenly are even weaker to that damage type than usual
