I faced with flashcards bug with the option “Remove flashcards from this session after they have been successfully tested two times in a row” checked. When I click “Got it!” the numbers of words on the right of slash and on the left on slash change simultaneously.
I think that’s a feature rather than a bug, in a way like “as for the words you have in your deck right now, card number 9 of 13 is showing” - as soon as you have successfully checked one card twice the number of cards in the deck decreases (as it should). I’ve used it as an exercise to lower the number from 25 to 0. I takes some time, though.