I'm using android.
There are 3 pages for 3 decks. When I quitted the game after using the deck on page 3 and opened the game again, my deck on page 3 changed to the deck on page 1. And I have to rearrange my cards again.
It happened like 3-4 times. I didn't remember the specific of the first 2-3 times. But I think it might duplicate the deck on other pages too. I was too confused and hurried back then.