When you copy in a deck from a deckstring, you can have some cards you do not own yet.
I’ve found that for duels (might be normal or just heroic duels), when you craft a card from clicking on the card to craft it makes an error sound but still successfully crafted the card.
If you do this for all remaining and try to back out, it still shows you have missing cards but considers them crafted. From here on until restarting, I find that returning to duels to deckbuild has many cards missing and you can no longer paste in a deckstring.