We haven't added account creation and login yet, so your flashcards aren't going to be saved long term. That means if you install the app on a new device, or re-install the app on your existing device, your flashcards will be gone.
Also, when we make changes to our language database (to add new content, for instance), that may also wipe out your flashcards.
For the time being, use the beta to play around with the app, but don't go crazy with card creation, as your flashcards are definitely going to disappear in the next few weeks.
One of our top priorities is to add login as soon as we can, so that you can comfortably make flashcards, knowing that they'll stay with you forever.