Some items are intentional not to be upgraded, regardless of smithing perks
Right, except for the part where I make a Legendary-smithed ancient nord bow into a Nord hero one at the Skyforge, and lose the Legendary status and wind up turning it into one of the weakest bows in the game. I'm really sure that's intentional, considering all the other nord hero gear is improvable.
There's even a loading screen blurb that say every piece of gear in the game is upgradable.
Horned Scale armor?
Imperial Studded Leather?
The smithing mechanic is broken. Not glitchy, not on again, off again, it's flat out broke. It amazes me they haven't lifted a finger to fix this.