I have now tested and confirmed this bug twice after multiple items have gone missing from stash, inventory, and even off of my character while wearing them and I think I have the issue nailed down.
I’ve been actively searching and trading for improved gear/stats, and what seems to happen is if you have a newly found or traded for base item that is nearly identical to the one you are currently using (or at least similar before it was tempered or master worked) the game will automatically get rid of one of the items. I have now had this happen multiple times.
Today, I logged on as my eternal softcore Spiritborn. I traded with a friend of mine for a pair of gloves that was close to identical to my existing item (3GA gloves with Attack Speed, Max Life, and Quill Volley). At first the trade wouldn’t put the item in my inventory from the trade window, so I placed my current pair into my stash and completed the trade. I favorited the new pair of gloves and put them on my character. I logged out to eat lunch, and when I came back the item I had placed into my stash was gone. That pair of gloves was also favorited and I did not visit the smith or stores. I was literally logged in for maybe 5 minutes and did no other activity. This has now happened to me with a Helm, and pair of gloves. I tried contacting Blizzard support but they said they can’t help.
I’ve seen this reported before in the forums, but clearly this bug is still out there somehow. Can someone from Blizzard please try replicating this scenario to try and trace? It is maddening to put in huge chunks of time trying to organically grind and find the items needed, or by trading other high-value finds for what you need only to be left with nothing. When you have a high-powered build and you are intentionally looking for nearly identical equipment where you want a better tempering/masterworking result, this is a huge risk as it seems the nearly identical items do not play well with one another!
Someone had suggested that perhaps one of the items was hacked/duped somehow that causes the issue, but then I’d have to ask how hacked/duped items are happening and still allowe. Please, can we find some way to address this?