It's intentional.

All the shisui sets (along with the faire joi) share the exact same model, merely with texture manipulations in order to hide/show certain parts of each piece, which can be gleamed with online FF14 model viewers. This is a problem since some of the shisui sets have a cloth neckpiece around the neck of the character, which would clip with many necklaces of the game. The other problem is that due to the setup, if they changed the metadata to allow necklaces to show on the fair joi, it would cause them to be visible on every single Shisui set, including the ones with the cloth neckpieces.

As a result, it is disabled to account for the neck cloth versions.