I've seen a past thread about this issue from three years ago, just around Stormblood's artbook where they first introduced this limitation, and the official statement was that they were going to continue looking into it.
It's been two more expansions and many more artbooks / soundtracks since then and it seems there still isn't any change in sight.
This is extremely frustrating. I just don't understand why you would make a unique item like this limited to one character. On top of that, even if I were to buy another artbook or soundtrack, I wouldn't be able to redeem the code again.
As someone who plays two characters ( one on NA and one on EU ) it sucks that I paid money for something, but can't have it on all characters no matter what I do, including spending even more money for it.
Even more frustrating that they said it's because their account wide items are limited, yet continue to release new account wide mounts etc.