Got reply from Feedback Assistant. Apple has fixed their backend so that purchases on apps with duplicate Bundle IDs will no longer fail. This issue has been resolved.

Thank you for fixing this issue, and it seems that no error has occurred since 7/22 0:30. Could you tell me about the detail of the cause and fixes, as I can take measures for the future.

Thanks for the information. It seems that I also have 2 duplicated Bundle IDs (aaa.bbb.cccDdd, aaa.bbb.cccddd). The second one is not used, but I couldn't remove it. An error says that the App ID 'aaa.bbb.cccddd' appears to be in use by the App Store, so it can not be removed at this time. Could you tell me how you removed the duplicated one specifically?

Seems to be the same situation (SKProductsResponse does not return valid products on iOS 15.4 or higher). Contacted with Feedback Assistant, but still not resolved and the error has recurred today.

Thanks for the reply. This issue occurs only in Production App. On the other hand, I can perform purchase in Sandbox or TestFlight with no problem. Also, this issue has started to occur since the end of March this year, and occurs and converges irregularly since then. I have four Product IDs in a subscription group, and cannot purchase with anyone.