On the third chapter of the Eldest Reborn if the player chooses a transform card to return to the battlefield normal form is put on to the battlefield while its transformed side is put in the hand. The transformed side in hand has no mana cost assigned to it and can be exiled.
Any card that “steals” transform cards from your opponent is returning the flipped side. Some cards (well, one that I know of) even gives you both sides in hand. Very strange bug indeed.