@Vashzaron I think you might be on to something. I have no evidence for this because I'm not interested in exploits in an online game because of the way it can damage the virtual economy, but I do have a theory. If any information about items on the player shop is retained in memory while managing a given shop, theoretically that memory buffer could be overloaded by throwing too much data at it so that the stored information can be manipulated. In theory, this could have any number of impacts on the data stored including changing the item ID to something different entirely. If what they're doing is something similar to this, surely the shops could be modified to simply prohibit the function used to do the exploit so that it is no longer possible. All of this is, as you say, spitballing. But that is a possible explanation. Those jukebox stacks could have been a player overloading the memory buffer to create new items. If I'm right, that means we've got both a duplication glitch and an item generation glitch that will both need to be patched. In any case, that explains where the meseta sellers are getting their resources.
It makes sense that this is a global issue. Sounds like it's something that was present and undetected on the JP side for a long time, discovered more recently, and then found to be also working on the NA side. It was never detected, so its present in both versions. That makes things extremely interesting. Because we're not up to speed with JP yet, each new release will need to be prepatched.