well i am guessing its an issue with save file, how do the save files work, would useing diffrent computers effect this, i need to know because i just won a Nymph from oracle and dont feel like losing that too.
The game saves every time you enter the main menu.
How to make your cards disappear:
1) Open Elements in one tab.
2) Open Elements on another page/pc/tab/etc.
3) Play a game in tab1 and win something.
4) Do something in tab2 that will save the game (visit the oracle, play a match, visit bazaar/deck menager and back to the main menu).
5) Your game has saved progress on tab2 removing whatever you did in tab1.
That is not a bug, it's just how auto-save works.
Using different computers should not make your items disappear unless you are using both at the same time and on the same account. For your own good, don't do it