When I play Possession on my opponent, and have finished the extra turn with their hand, for a short moment I can see my opponent's next hand. The cards seem to fade in before the switch to my own hand happens (with the black castle screen). It's also in the log.
I have attached a screenshot.
Sorry, this is not a bug!
That is ok according to rules. You are posessing your opponent even during cleanup phase, when he draws next 5 cards.
Oh, you're right! Thanks for the hint, didn't know that.