Just played a game with Ambassador. It worked perfectly when trashing/giving away coppers. But not for any other card I tried. My opponent had the exact same issue. Worked fine with coppers, but not for anything else. Hmmmm
What other kinds of cards did you try?
Note that cards that aren't in the supply (like shelters) can't be returned to the supply, therefore revealing them with Ambassador does nothing.
Revealing the shelters provides information if I'm card counting. It's unlikely situation, but I could see trying to pull off some combo like village, masquerade and transmute if I knew that an opponent's hand was 4 treasuries and an overgrown estate but wouldn't if it was four treasuries and a hovel.
(Error occurred on game #463695 on oregon Turn 9 for the bot - didn't impact my strategy, but did go sending me to the forums to see if it was a known error)
It's unclear to me what the error you are talking about is
Upon playing Ambassador, the player must reveal a card from their hand. The game does not always reveal a card.