If Enchant Weapon has the Kindling Blade upgrade and enough levels of Efficacy to reduce the mana cost to 0, it fails to recast automatically when you’re at negative mana. Instead it waits until you have positive mana.
I’d expect that a 0-mana ability could, much like any other, be cast when in the negatives. And in fact, this one can. But it still fails to automatically recast.