I don't think you can disarm him. I killed him today and the last hit was a successful disarm, but he kept his staff on him.

Disarm as in 'drop the weapon' or 'put the weapon away'? If you kill things with disarm, I don't think the unequip always happens. I've also had successful disarms do nothing at all if an NPC uses a feat or casts a spell.

Additionally, as far as I know, NPC's won't ever lose their items for you to pick up, if that's what you meant. >_>

So if the item isnt marked as lootable you cant gain it even with a disarm?

Makes sense I guess, though the disarm should still make them unable to use the item for a time.

If you check any creature in the toolset, you see that you can check several boxes in the "Advanced" tab, one of which being Disarmable.

My humble guess is that this particular box simply isn't checked.

So... is this indeed a bug at all?

(I suppose they still "put the weapon away" when this is not checked, but like Strife said maybe he was just too busy dying.)

Should be fixed.

You're not going to get the weapon that they're wielding under any circumstances, but you should be able to force them to put it away.