And whether they're working as intended or if they're considered bugs/oversights. These are just a few things I noticed in my playthrough (playing on IWDEE v2.5)
A number of resistance spells give magic fire/cold resist despite there being no magic fire/cold attack type. There's also no flavor text mentioning magic fire/cold in the spell descriptions like in the BG games.
Bears (shapeshifts and summons) are immune to web. I was playing both IWD2 and IWDEE at the same time and bears are not immune to web in IWD2. I also checked BG2 and they're also not immune. Not really an issue or anything but thought it odd that this differs from the other IE games.
Cursed weapon in offhand, specifically the Breath of Auril dagger. I noticed when putting this in the offhand, you can switch your main-hand weapon to a ranged or 2-hand weapon and not get the effects from the dagger. Seems like equipping in the offhand should prevent weapon-switching like it does when equipped in the main-hand but again, I'm not sure what the intended behavior is here.
2-handed weapons with spell casts. So normally all spells from weapons in a character's quick weapon slots show up in the Use Item menu when you click it. With 2-handed weapons (like various staves, Storm Bow, Icon of Power etc.) if the character is currently holding a shield or offhand weapon, the spells from the 2-handers won't show up in the menu. Remove the offhand item and the spells will appear. Note that the opposite isn't true. If a shield/offhand weapon has a spell, it still appears if currently holding a 2-handed weapon.
Shields of Lathander don't block status damage (poison etc.). I think this is fine for game balance, but the spell description says "immunity to damage" which reads like it should include damage from statuses. In IWD2, these spells specifically increase your damage resistance stats so no status immunity is expected.
Invisibility and enemy behavior. When the entire party is invisible there are a couple areas where enemies will home in on and surround the party leader. This happened a few times but the one I remember is in Castle Maluradek with the enemies that spawn when you walk past any of the guards. The enemies aren't actually seeing your characters and won't attack. They just crowd around the party leader. If any characters become visible, the guards will see them immediately if in range. Of course, invisibility in general is an issue for the enemy AI but this particular behavior I felt was worth mentioning because it's kind of strange.