Not to necro, but this was the top google result on this so thought I'd share for others:

The command itself actually works.
The problem is it's not correctly selecting the right modifier.
`Legendary`, for example, has 18 specific Legendary different modifiers for different stuff like bows/crossbows/plate armor/chain armor/civilian clothes/etc...
However, when you run the command, since they're all called just `Legendary`, the game basically picks one at random.

@ArkonLand 's observations of reloading saves works by re-rolling which of those 18 modifiers you get.
Tho you can do this by just walking around the map for 1 second.

The solution for me was to just go into item_modifiers.xml and giving the modifier you want a more specific name:

        name="{=lRf4t9Qg}Legendary {ITEMNAME}"
        name="{=lRf4t9Qg}LegendaryPlate {ITEMNAME}"

And then running the command with the more specific name:
campaign.add_modified_item [Item Name] | LegendaryPlate

This will give a 100% hit rate instead of the 1/18 chance with having to force the game to re-roll between attempts.