iOS auto-renewing subscriptions restoring updates transaction to SKPaymentTransactionStatePurchased when enter background

Question

In sandobox, I'm testing auto-renewing subscriptions of in-app purchase (iOS 7.1, iPhone 5).

1. Call "[[SKPaymentQueue defaultQueue] restoreCompletedTransactions]".
2. Enter password.
3. immediately enter background by tapping home button.
4. Restart the app.
5. "- (void)paymentQueue:(SKPaymentQueue *)queue updatedTransactions:(NSArray *)transactions" method called and its transactions' transactionState are SKPaymentTransactionStateRestored.
6. "- (void)paymentQueue:(SKPaymentQueue *)queue updatedTransactions:(NSArray *)transactions" method called again and its transactions' transactionState are SKPaymentTransactionStatePurchased.

I don't expect 6. process.
Is it sandbox bug or can I fix it?