Item on cursor destroyed

Bug report reference number: 3,445,570,421

I got very unlucky and accidentally deleted 2 House of Mirror cards just now.
It happened when someone made a trade request for them and joined my hideout, I picked up the stack of 2 cards from my stash and dropped them in my inventory (actually Wildwood Rucksack) and near-simultaneously hit my Awakened PoE trade hotkey which does "/tradewith \@last".
At that point, they just disappeared, and I was very confused, then I noticed in the chat log it said "Item on cursor destroyed.". When I tried to reproduce this with a low-value item afterwards, I could only get the one of the two expected results: "You cannot trade with an item on the cursor." or the trade going through, so it may require precise timing. (It looks like the person I was trading with hadn't quite entered the zone yet, and loaded in just a moment later, so that could be a factor.)
I have no weird hotkey bound to "/destroy" or anything (the adjacent hotkeys are harmless /invite and /kick FWIW), so not sure how this could happen without it being a bug.
I've noticed there is an invisible (about 1 pixel) gap between the rucksack and the main inventory where attempting to place an item there will pop-up the "Destroy item?" box (and also note that pressing return at that moment - like a trade macro might do - will carry out the "destroy" option!), but in this case I was clicking to place the item centrally in the top-left square of the rucksack.

(Screenshot link: https://imgur.com/a/SJlKVdW)