Card count is weird

So I have 2708 cards in my collection

If I filter by rarity, selecting all rarity except PROMO, the game tells me I have 2618 cards

If I filter by selecting all the extensions instead, the game tells me I have 2617 cards

Which means that there is 1 card whose rarity is NOT PROMO, but that is not in any extension. Which is weird.

I also can't find which card it is.

If I filter only promo cards, I find 90 cards, which is consistent with the hypothesis. But doesn't help me find the mystery card.