Completed the task for heroic tavern brawl and was awarded 3 packs. Opened first two packs as normal, tried opening third pack, it blew out and disappeared. No cards, nothing.
Game froze in card opening screen. Had to ‘quit’ game. Restarted game, no pack.
If you look closely you’ll see that you did receive the cards.
It’s just a visual bug; you were not cheated.