I'm not sure if this bug is already fixed but it's definitely worth mentioning...
I have noticed that in the vanilla BG2 game I have a few items that give Magic Resistance for the NPC and all add up. Trouble is, they don't stay stacked after a while and in order for the bonuses to add up again the items would have to be re-equiped. This happens after loading a game, going to a new area or just plain resets after a while.
CURRENT BEHAVIOR: Carsomyr: +50% MR Kaligun Amulet: +10% MR Ring of Gaxx: +10% MR TOTAL: 70% MR in total but resets to 50% quite often. (Due to Carsomyr being dominant)
EXPECTED BEHAVIOR: Carsomyr: +50% MR Kaligun Amulet: +10% MR Ring of Gaxx: +10% MR TOTAL: 70% MR in total and stays stacked through the game.