I assume you are in a org-fc review, then navigate to a card different from the current one and use the suspend key binding (bound to org-fc-review-suspend-card)?

The behavior you're seeing is due to an oversight in the design of this function, because I didn't think that people would interact with other cards during the review.

This should not be hard to fix, instead of using the current card of the review session (which might be different from the card at point when using the suspend function), we can get the ID of the flashcard at point and remove that from the session.

The best I can do is change org-fc-suspend-card to remove the suspended card from a review session, if there is one.

Because it needs to continue the review, org-fc-review-suspend-card should only be used on the card that's currently being reviewed, other cards in the same (or any other buffer) can be suspended with org-fc-suspend-card.