Load up the Art of War: Late Siege challenge and use a Mangonel to attack an archer on the wall. You will find that if it’s been recently packed, it does not respond at all. To reproduce this, tell the Mangonel to move, wait for it to pack, then tell it to attack a unit, the Mangonel will not respond to the attack command.
But if you wait about 20-30 seconds after it packs, then it will respond. Time seems to be the only factor here.
Attack ground works fine. Or if you unpack it first, then it will respond every time.
But packed Mangonels won’t respond to a right-click attack command nor an “A” attack command for 20-30s.