Spinal Blades
Lootballs – Elementalist
Not sure this is a bug, as they clearly stated when created that if they were transmuted, they would not be able to be used in the Mystic Forge.
“Can be combined in the Mystic Forge with Blade Shards, Luminous Dust, and Steel Ingots to create a Twin Spinal Blades back item. NOTE: Cannot be used in the Mystic Forge if transmuted.”
Unused Upgrade Slot
— In-game description
Maybe more of a suggestion?
If you only changed the skin they still work in the forge for upgrading.
At least they did when I tested when they came out. I don’t see why anyone would have gone out of their way to change how the forge works just for these items so it should still be the same.