Pretty straight forward, cannot see them, cannot hit them, unless auto attack kicks in, then can hit the one I am fighting. Can still see chosen of the normal type, just not the new ones in Lower/sewers.

This was mentioned before, if you have any kind of model override it can cause this to occur.

Sometimes NWN just bugs out, too, in which case a relog will usually fix the problem. Happens for other monsters as well.

Not a bug.