stripe: problems setting connected account on subscription

I'm having some issues for adding an amount fee on a subscription on Stripe. Looking at this documentation I should be able to add a connected account when creating the subscription so i can set an application fee on the subscription without having to do any modification on it afterwards.

I'm trying to use a RequestOptions on the subscription creation to add the connected stripe account but it seems that if i use it it's redirecting to the live account and it's not finding the customer i'm using making the request to fail.

Here is the code of the whole process:

        attachPayMethodToCustomer(bookingDetails.getPaymentMethod(), user.getCustomerId());
        Price price = getPriceForItem(item, bookingDetails.isExclusive());

        SubscriptionCreateParams.Item item = SubscriptionCreateParams.Item.builder()
                .setQuantity(1L)
                .setPrice(price.getId())
                .build();

        BigDecimal bd = new BigDecimal(calculateSubscriptionFee(price.getUnitAmount() / 100, bookingDetails.isRecorded()), new MathContext(2, RoundingMode.HALF_UP));

        SubscriptionCreateParams.Builder params = SubscriptionCreateParams.builder()
                .setCustomer(user.getCustomerId())
                .setApplicationFeePercent(bd)
                .addItem(item);

        if (bookingDetails.hasExtras()) {
            Price recordPrice = getExtrasPrice(seller().getCurrency());
            SubscriptionCreateParams.Item extraItem = SubscriptionCreateParams.Item.builder()
                    .setQuantity(1L)
                    .setPrice(extraPrice.getId())
                    .build();

            params.addItem(extraItem);
        }

        RequestOptions ro = new RequestOptions.RequestOptionsBuilder()
                .setStripeAccount(seller.getStripeAccountId())
                .build();

        Subscription subscription = Subscription.create(params.build(), ro);

I'm receiving this exception when it tries to create the subscription:

com.stripe.exception.InvalidRequestException: No such customer: 'cus_IaFZjrBAlBsXXX'; code: resource_missing; request-id: req_C67JN4tgneXXXX but i can see the customer on the dashboard. I tried also to override the stripe api key to use the test one, but i have the same issue.

Thanks a lot, Adrián