This is a bug that’s been noticed multiple times by different users on the forum within the last few weeks: Occasionally a card will not display correctly in a player’s hand. The card is in the player’s hand, but cannot be seen.
Here it is happening to Greg (note that the count at the top says he has ‘1’ card in his hand, but there’s is no icon showing the card in the sidebar (when Greg took his turn, the in-game animations showed mana flowing into the invisible card in his hand from his swap)
Here’s another example, in a video posted by another player; you can see from the battlelog that the player had a creature in play. which was returned to the player’s hand with a Perilous Voyage.. the video starts just after this point. There’s no sign of the creature, a Sunbathing Rootwalla, in the player’s hand, until the player rearranges the cards in their hand and tries to drag a card to the very bottom of their hand, at which point the Sunbathing Rootwalla reappears.
‘Invisible’ cards which ‘reappear’ when you move cards around in your hand is a bug which I’ve seen previously in this game; however, up until now, I’d consider this bug to be extremely rare, but it seems to have become much more common, presumably as a result of a recent update.

