The two most frequent reasons why items disappear are the following:
1) Accidentally selling an unequipped item. When making transactions in your inventory it is possible to sell your item unintentionally
note: To avoid selling an item by accident, you can lock it. Tap on the item and then on the padlock symbol, which will turn from grey to white. You will see that the “Sell” button is now greyed out and it is not possible to sell the item by accident anymore. To unlock the item, simply click the padlock icon again.
2) The item is not an item, but a Cosmetic. You can receive cosmetics without owning the piece itself. To find out more about Cosmetics, check this article.