It's not a bug. It's because Square Enix doesn't want to update the game to handle purchase receipts signed with SHA-256.
Crystal Chronicles was released in 2020. The game is essentially one giant collection of free demos, with IAPs to unlock the specific games inside. Square opted to do on-device verification of IAPs (which is generally a good thing), which means the app relies on purchase receipts sent over by the App Store.
Up until 2023, these receipts were signed with an SHA-1 certificate, at which time Apple started a process to migrate over to a much stronger SHA-256 certificate. Between then and the end of last month, both certificates were available. But on January 24th, Apple moved to the last phase of the migration as the SHA-1 certificate expired.
As a result, the current build of Crystal Chronicles cannot read Apple's modern receipts. It would need to be updated to handle the SHA-256 receipts. In fact, Square can't even submit a new build of the game to the App Store without SHA-256 support, as that has been a requirement since August of 2023.
There are a few other ways around this as well, though all would take more effort on Square's part. Apps that rely on server-side verification weren't affected by the certificate change (that's all handled by Apple's servers). And there are now functions in the StoreKit API such as Transaction that fully offload the cryptographic process to the OS, abstracting away these kinds of changes. Though I don't believe this was available in 2020 when the game was first released.
Dev-wise, there may be other things that Square would need to do to bring the game into compliance with current App Store rules as well. The minimum SDK version Apple accepts right now is iOS 17, for example. So the project would need ported from what I'd wager is the iOS 13 SDK. That shouldn't be a big deal for a well-written app. But a poorly-written, minimum-effort port - especially one making heavy use of external libraries - could definitely have issues.
Ultimately, Square kind of tried to have their cake and eat it too, and it backfired on them. Had they not relied on IAPs, this issue would have been avoided. Alternatively, if they had gone fully in the other direction and used server-side verification, this issue would have been avoided. But the on-device route, while noble in some respects, also comes with the most maintenance work, which they are now opting out of doing.