Turn 6. Played PD. Got card + action as normal, but saw no option to put bottom card onto top of deck. Game log shows that I received a copper and that I looked at Potion (ie, the bottom card on my deck). But NO window giving me the choice as to topdeck or not.
Later, on Turn 9 (and other turns after that), the game DID work properly, and there was a pop-up window, asking me if I wanted to topdeck or not.
No idea why it would not work on the first [applicable] turn of the game, but would work properly after that.