So why can enemies (regular humanoids) target my character (before they even act in a round) with Rings of Invisibilty, when I can't target a Master Thief with the same Ring?
Known bug, or "undocumented design feature"?
Did you just put the ring on, or have you worn it through a previous fight? I think you have to recharge the Invisibility spell by taking the ring off and putting it back on, because it wears off like the normal Mage spell.
Hmm...that's weird, 'cause the other effect of invisibility - being able to move away from melee without provoking opportunity attacks - always work (unless enemies can See Invisible). And
If I have to re-quip the rings after each combat, it will be like the Buck Rogers games where you had to reload the heavy weapons (grenade launcher and plasma throwers) after each battle manually, which was rather tedious.
Anyway, I just tested, and the enemies can still target my characters with spells, even if my character has unequipped and ree-quipped the ring, and not acted yet in a round.
So it looks like Invisibility is bugged.
I haven't tried the Invisibility spell or potion, though. Maybe that will work differently.
EDIT: looks like you can move next to an enemy on Guard, and not trigger it if wearing a Ring of Invisibility.