The ALCHEMY card Thought Partition revealed two copies of the same card in my hand. Its controller chose to make one of those cards white and perpetually five mana. I then drew a third copy of the same card, and cast one of the copies that was not 5 mana. The arena interface indicated that all the cards in my hand had been revealed except for the one that had been made 5 mana. The correct thing to show was that the remaining copy of the card that wasn't five mana was an unknown card.