Saw this bug twice today. Shadow step not decrementing cost of void ray. It was at one when I played it and it returned to my hand at one.
Cost reductions only apply while the minion is in your hand/deck. Minions do not retain cost reductions after they enter the battlefield.
So after you played the Void Ray for 1 mana, its cost on the battlefield reverted to the usual 3 mana.