The only thing I can think of is that I used to have a card of which I exchanged the last one to a friend because I was wondering what happen when you exchange the last a card you only have one of.

Long story short, the game still show the card's image in the overview and considered the card obtained, but it says you don't have it in the specific card's view.

Maybe that's the reason for the weird behaviour. I don't know how to test it, apart from obtaining a copy of the card again.

Edit: hohoho, interesting indeed.