Its hard to tell how to reproduce this bug because it happens randomly. If you store cards on a bag, and play on the table for various days overwriting the table to keep progress there are random times where the bags get bugged in a way when you try to take out one of the items stored (mainly cards or decks) they just dissapear completely like getting deleted.
I can kind of guess this is some sort of anomaly caused when the game creates or modyfies the save, there is for whatever reason an odd that the cards (or other items inside a bag) to get bugged inside making on the next load the game believe there wasnt actually such item inside and automatically deleting it to avoid some sort of crash or error.