Got a reply from SE support, but it's not good news and implies all future bonus items will also be one character only:

Thank you for contacting the Square Enix Support Centre.

In response to your recent query regarding the use of item codes on your account,

We would firstly like to apologise for any confusion on the use of codes or if you have felt misled at any point. This is not our intention nor our aim.

However, we cannot change the wording on the Store or influence this. If you are looking to obtain a refund, please reply back to this message so we can arrange this. Having said that, we would like to confirm that the codes will continue to only apply to one character and this will also affect the Wind-Up Yotsuyu minion code included with your other order.

Finally, if you have any feedback relating to this issue, please post this on the Square Enix Official Forums.

If you have any other queries, please contact us again. We apologise once more for any inconvenience caused.