I first noticed this problem fighting a named mob in hell cows, but it also occurred after a Duriel fight, which I imagine means it’s much more widespread.
I have max resistances. When cursed by named mobs (apologies I don’t know the exact curse), I take a 28 percent hit to cold resist and a 36 percent hit to poison resist. When the curse/frozen damage lifts, the negative modifier remains at the the lowered levels (47% and 39% respectively). Cleansing does not have an effect. Going to town NPC’s that fully heal the player has no effect. Resistances remain lowered until the start of a new game.
I apologize if I’m wrong and this is intended, but it doesn’t seem it would be. Hope this helps and happy holidays.