Anti wards after purchasing them and then running into wards you then get the expired message as expected. Come next round though it stays in your inventory but is inactive so it doesn't work on wards from then on. You cant but another one until you over write it (because you already own one) with a different item(have to have your inventory capped at 3 to even do this)
So in summary anti ward either needs to reactivate come next round or be removed from your inventory so you can purchase another one again. However its supposed to work. (Think it used to be removed back in the day)