Crimson Longinus
Legend
So this is an example that strikes me as odd because it is a quality of the character... it has a lampshade akin to the one we apply to spells.
Yet many folks would classify this as a narrative mechanic, but would not do so for spells.
I guess it depends on how you assume magic to work in-universe, but how I'd interpret these things, they don't look the same.
To me this retroactive creation of an item is clearly a narrative mechanic. It brings an element into the fiction by breaking temporal causality. It also is very meta as the character cannot knowingly use this ability or know its usage limitations. I don't see anything like that being required by D&Dish magic.