More a quirk than a bug really.
The game engine was never designed for players editing their equipment's descriptions in-game, and it has absolutely no way to make the distinction between different descriptions for a same item. In a nutshell, each item has a resref (resource reference) tag, being the name the system knows the item as. When you stack items that have the same resref it simply adds the newer items to those that were already in your inventory, keeping the description of the original item.
Definitively here to stay like this forever.