Harpoon bug

I have a poisonous attack spear that also poisons enemies in combat. However, when I harpoon a mob that is immune to normal harpoons (a certain magic globe), but not to poison attacks, it is still immune to my poison harpooning. Shouldn't the attack type of the spear being used to harpoon be relevant? Also, I get the impression that when I harpoon with the poisoned spear, it isn't actually getting a chance to poison. Might be wrong on this though.