My app has a single subscription group called Premium that contains two products with different billing periods - monthly and yearly. All tests are made in Apples's Sandbox environment with sandbox user.
I am currently testing a scenario, where the user is subscribed to the monthly product, subsequently cancels the subscription from the App Store subscriptions page, and before the subscription expires, he wants to re-enable it (same monthly product). If the subscription is re-enabled from the App Store subscriptions page, my server gets a notification about the changed subscription renewal status and everything works well. This scenario is documented at the bottom of the page here: https://developer.apple.com/documentation/storekit/in-app_purchase/testing_in-app_purchases_with_sandbox/testing_disabling_auto-renew#3780478
I however also want to support the scenario where the user can re-enable the subscription before it expires from the app itself. To do so, if the subscription is cancelled, but not yet expired, I show the user a button Re-activate. When this button is clicked, I initiate a purchase of the same monthly product, the IAP correctly shows the payment sheet and once I click Subscribe in the In App Purchase payment sheet, it goes through without any issues. However, my app subsequently receives a callback that the purchase failed:
<SKPaymentQueue: 0x280f1fa10>: Payment completed with error: Error Domain=ASDErrorDomain Code=500 "Unhandled exception" UserInfo={NSUnderlyingError=0x2803a4720 {Error Domain=AMSErrorDomain Code=301 "Invalid Status Code" UserInfo={NSLocalizedDescription=Invalid Status Code, AMSURL=https://sandbox.itunes.apple.com/WebObjects/MZBuy.woa/wa/inAppBuy?REDACTED, AMSStatusCode=500, NSLocalizedFailureReason=The response has an invalid status code}}, NSLocalizedFailureReason=An unknown error occurred, NSLocalizedDescription=Unhandled exception}
This seems like the purchase failed with invalid status, but strangely enough, my server receives a notification that the subscription renewal status was changed to AUTO_RENEW_ENABLED and if I check the App Store subscriptions, I can see that its not cancelled anymore. The subscription also gets renewed at the end of the billing period, where it would have otherwise been cancelled. So in other words, everything seems to work except the purchase error above.
My question is, what could be the reason for this? Perhaps Apple does not support re-enabling subscription directly from the App, but only from the App Store subscription page? Or perhaps its just a limitation of Apple's sandbox environment and I would not receive this payment error in production environment?