I use ExamplePaymentQueueDelegate to restore purchases on iOS, and it works fine in my case, in promo codes as well.

  Future<void> init() async {
    await _fetchSubscriptions();
    final purchaseUpdated = _inAppPurchase.purchaseStream;
    _subscription = purchaseUpdated.listen(
      _listenToPurchaseUpdated,
      onDone: () {
        _subscription.cancel();
      },
      onError: (Object error) {
        log(error.toString());
      },
    );

    if (Platform.isIOS) {
      final iosPlatformAddition = _inAppPurchase
          .getPlatformAddition<InAppPurchaseStoreKitPlatformAddition>();
      await iosPlatformAddition.setDelegate(ExamplePaymentQueueDelegate());
    } else {
      await InAppPurchase.instance.restorePurchases();
    }
  }

class ExamplePaymentQueueDelegate implements SKPaymentQueueDelegateWrapper {
  @override
  bool shouldContinueTransaction(
    SKPaymentTransactionWrapper transaction,
    SKStorefrontWrapper storefront,
  ) {
    return true;
  }

  @override
  bool shouldShowPriceConsent() {
    return false;
  }
}

Also, please show price consent on hitting subscribe button to avoid future bugs

    if (Platform.isIOS) {
       await confirmPriceChange();
    }  

Future<void> confirmPriceChange() async {
    // Price changes for Android are not handled by the application, but are
    // instead handled by the Play Store. See
    // https://developer.android.com/google/play/billing/price-changes for more
    // information on price changes on Android.
    if (Platform.isIOS) {
      final iapStoreKitPlatformAddition = _inAppPurchase
          .getPlatformAddition<InAppPurchaseStoreKitPlatformAddition>();
      await iapStoreKitPlatformAddition.showPriceConsentIfNeeded();
    }
  }