When using the card Vanish to return all minions to their owner's hand, the cards remain on the board just turned over. It blocks the view of the opponents cards played afterwards.
I have also encountered this issue. Moreover, I was unable to select the cards the were "returned" to to my hand as they remained in the play area and not in my hand. I noticed that their were holes in my hand where the cards were supposed to be and could select some of them from there :/