Shoulder enchant for heirlooms

I've seen copious amounts of information regarding the use of the Zandalari enchants on heirloom shoulders existing. Basically the heirloom enchant is 55 and the enchant won't take effect until the wearer then becomes 55.

Is this supposed to work here too and it doesn't? Is it a bug that it doesn't? The enchant doesn't say anything about item level and the information I've seen always mentions it being able to be enchanted via one toon and sent to another and the effect will work once the wearer becomes 55.

Just to be clear, you have applied the enchant to a shoulder heirloom item, you have equipped it with your level 55+ character, and you don't get the enchantment effect?

No,

It won't allow me to put the enchant on it at all saying it's "not a valid item".