That's the case in every mechanic basically so nothing new there.
I think what's rustling me about this is that is just absolutely shits on you for having a character invested in defenses. My fortify/max res/armor/evasion wheels, determination, banners, all of it means jack shit in sanctums. None of it reduces my resolve damage at all, and thus far in act9 the actual HP damage in them is so low as to be basically 0.
So i've got multiple auras and 30ish passive points just doing absolutely nothing for me in sanctum content. I've stopped even trying for now, maybe give it another go on a 2nd character that's a 2k life ice shot deadeye or something retarded that is more suited for it.