Confirmed, and with equipment it's worse than with stacks. The inventory window already hands the drop handler the exact copy you dragged (its instance id — the thing that carries that item's scrolled stats and remaining upgrade slots), but the handler ignores it and calls the key-wide remove, which pops the FIRST equip in the bag with that item id. So if you own two Blue Bandanas and drop the plain one, the game deletes the +scrolled one and leaves the plain one sitting in your bag.

The fix is one line in the drop handler: when an instance id came in, remove THAT instance (InventorySystem.removeInstanceByIid already exists and is used by storage and trading for exactly this reason); otherwise fall back to the slot position, and only then to the old key path.

One related thing I want to flag while I'm in here, because it also loses stats: whatever copy gets dropped, the item that lands on the ground is spawned from the item id alone, so its scroll bonuses do not travel with it — picking it back up gives you a fresh, unscrolled one. That needs the ground drop to carry the instance too, and I've noted it as a follow-up rather than bundling it into this fix.

Confirmed and fixed for equipment. When you dragged one of several identical pieces out of your bag, the game dropped the right ITEM but removed the wrong SLOT — it always took the first (top-most) slot holding that id, so the icon that disappeared was the top one, not the one you grabbed. Worse, with scrolled gear that meant you could drop a +7 copy while thinking you were dropping the plain one. Every piece of equipment in your bag already carries its own unique id and the inventory window was already handing it to the drop code; the drop code was simply ignoring it. It now removes exactly the piece you dragged. (Stacked non-equipment items — potions, stars — are the same bug from a different angle and are tracked under #139; they have no per-piece id to key on, so that half needs a bigger change.)