You can not disarm these guys with a successful Hit. They keep their crossbows out.

I notice many NPCs do this, I assume it is because the weapon has some sort of special property that is not meant to be dropped when the gnoll dies. (Some weapons do drop after successfully disarming them to their inventory)

Never assume!

What it is, is two functions in NWN.

One function determines if a foe *can* drop a weapon. That's set in the inventory property of the NPC.

A second function, on the NPC determines if it *can* be disarmed.

If both are set, the NPC drops the weapon on the ground when disarmed.

If only the function letting it be disarmed is set, the item drops into the NPCs inventory but does not drop on death.

If only the Drop Item property is set, then the NPC can not be disarmed-but when it dies, it drops the weapon.

Most Bioware standard NPCs are sadly set to be undroppable and undisarmable. Custom monsters can be set properly though so you can Disarm them, but not get them to Drop the weapon.