Hi, holding down A to pick up a card pile sometimes reveals cards in the stack that haven't been revealed and can't actually be picked up yet. I think this happens when the cards in question coincidentally are alternating colors off of the highest revealed card, but I am not sure. Hope this helps!
I've noticed this too, mostly when it's a covered card that you would be able to place the top uncovered card on. Like just playing a minute ago, I had a stack up to a black Queen card with the stack of covered cards underneath, and when I would hold to pick up that column, it would reveal a red King and move it off of that stack. Placing them in another location will leave the card hidden until it is uncovered normally.
That's great! One other bit of feedback - I think the timing for picking up the full stack of cards by holding A could be sped up a good bit. The Playdate's buttons are snappy enough, I think most people are going to be able to single-click for a single card without this causing any problems.
This one is more of a value judgment than a bug report, so I won't be mad if you disagree and keep it the way it is, but I thought I'd throw in my 2c just in case!