*** Small spoiler alert: The following content reveals a specific enemy type and their location in the early game. ***



My wizard went to clear the spiders in Deklo Grove. When the combat started, I had him casting Invisibility on himself. But the spiders always went to him and attacked him, even after he moved to a different spot, as if they can see him. The only difference I noticed is that Attack of Opportunity was not provoked when he was passing through the spiders.

Is this spell bugged?

So to be clear was your wizard attacking or casting spells that would affect the spiders?

If so this will negate the spell.

I checked the D&D 3.5 rules on the invisibility spell and it did note that invisible units are not silenced so you may be detectable due to the noise you make.

After I had him casting Invisibility and then doing nothing, the spiders can still went to him and attack him.

Well, the "noise" notion makes sense in PnP. But I doubt it was implemented in this game.

Pretty much EVERYTHING in this game is glitchy in one way or the other.