Queen of the Forest has disappeared from stash twice

The first time it happened, I blamed it on myself misplacing or selling it on accident, even though I doubted it. But now I know I put the Queen of the Forest chest plate in tab #9 in my stash, and it has disappeared again. I didn't sell it, trade it, lose it in my crafting bench, drop it on the floor. It's just gone. I don't know what happened.