Dunno if it's a bug or not, but i got drained 2 lvl by undead, but it doesn't affect HP or "TAC0" (i'm old school). Shouldn't it?

Nope, its working as intended.

I don't know what school you went to, but where I came from it was "THAC0" >_>

Mm.. Tacos.

There is no such thing as THAC0 in NWN. "To Hit Armor Class 0" has to do with (surprise, surprise!) Armor Class, and attacks.

Level drain's effect is hardcoded; the NWN Wiki does not indicate any known bugs or issues with the effect. I'm pretty sure this has been reported to BioWare, and that they have no intention of changing level drain.

It's working as intended - not that we could change it if it wasn't.