This is mostly for casting and archer monsters, but sometimes the melee ones pull it off too.
Mages can often cast at huge distances, typically beyond possible sigh range of a player. (I play at 1680 x 1050 and can't see them or even go to first person style and zoom to them) For the time, it's only been orc mages but I'm sure the AI applies to most caster monsters.
Archers tend to shoot at invisible people. This can happen by running past them or even if they were previously shooting you and you go invisible. While I understand that standing next to something invisible might not be realistic, they shouldn't be able to shoot and chase you when you move or run away. The default AI will not do this and I think it's some tweaked settings in the 'new' AI that is used here.
Monsters make listen checks too, they will chase you if you don't break contact.
The point remains, they are typically humanoids and their listen is not good enough, or should not be, to attack a large distances against an unseen target with only a 50% concealment penalty.
Attacking at point-blank range is valid, though the attacks at distance are a targetting bug as old as NWN.
The AI with casters and archers is so bad as it is. :(
I think Wild Orcs in particular seem to have a very large perception range. They are already charging to attack you before you see them on your screen in most cases. (Be it melee or ranged)
The AI is already horribly stupid. This far-range perception provides them a 'semblance' of intelligence.
As for shooting and chasing invisible people. Why shouldn't they? Invisibility is not a bullet-proof spell, combined with stealth and you can manage to move by unoticed, but alone? You should be wary before walking 1m away from bloodthirsty foes.
I think the archers shooting through invisibility is when they shoot you when you are invisible but not next to them, which is simply impossible without see invisibility or true seeing or some-such.
Perception range: this can be a real game breaker if applied to strong, ranged attackers. I do not know if all wild orcs have far perception range, but they certainly shouldn't have. I can't name a handful of EfU creatures with far perception because it is a big deal.
Listen: A long time ago (as in, close to two years), Arkov implemented an acknowledged AI system. This had a noticeable impact on game difficulty, but did not break anything. However, as of EfU:A I have personally witnessed creatures paying no regard to invisibility, presumably because of Listening checks. In and of itself, that isn't a problem, but for some time I've suspected we've either been rather liberal with giving our creatures Listen ranks or the system no longer disengages when PCs get far enough away -- this would be a problem.
However, also keep in mind that, on some quests, creatures are actively moving towards players. On these quests it can easily seem as if you have been engaged much sooner than you actually have.
They do not disengage when you further yourself from them, no.
Are the orcs arcane archers ?
Listen checks makes seeker arrows ?
When running from them in invisibility, hit the stealth button and they'll break contact.
Even playing characters with high stealth, 16-18 range, the orcs tend to never disengage. Simply put, I've never gotten away from one even using multiple items including Exped, Invisibility, Invisibility+Stealth, Stealth, Summons, or flat out running. None of them have acknowledged an invisibility or stealth once they've locked on. No matter the distance between them.
I out ran them once (invis+exped), hit a transition over half a screen away, loaded slowly, and loaded fugue on the other side. So while this may be perfectly reasonable. . . perhaps it deserves a second look.