If shopper updates payment method on a Stripe Billing subscription, it turns into a WooPayments subscription

[dmvrtx]

Describe the bug

Whenever shopper goes to My Account > Subscriptions > Change payment and updates it, e.g. by providing a new card number, Stripe Billing subscription gets cancelled and a WooPayments subscription is created.

To Reproduce

1. Activate "Enable Stripe Billing for future subscriptions" option in WooPayments settings. For it to be seen, the store should be in US and WooCommerce Subscription plugin should be active.
2. Create a test subscription and purchase it.
3. Check the subscription details in WooCommerce > Subscriptions. The subscription ID which starts with sub_ is an indication that it is a Stripe Billing subscription:

4. Visit the Stripe dashboard for the account. Subscriptions should be visible in the Subscriptions section:

5. Open My account > Subscriptions and change payment method for the subscription. E.g. update card expiry date.
6. Reload the subscription order as a merchant. Notice that the subscription ID is gone and there is a Payment method: Cards instead.

7. Check the order notes, which will indicate that subscription was charged with WooPayments:

8. In the Stripe dashboard, subscription is now stated as "Cancelled":

Tip

If Subscriptions section is empty, head to Developers > Events and look for an event which says that subscription was cancelled. From there, you can click on the subscription ID to load it.

Actual behavior

WooPayments sees that the new payment is a card and updates the subscription to be a WooPayments subscription.

Expected behavior

WooPayments should see that the new payment method is card, but also check the subscription type and update it appropriately.

[dmvrtx]

Findings after a brief look at the code:

• Stripe Billing subscriptions can be identified by having the combo of metas: _wcpay_subscription_id and _wcpay_billing_invoice_id
• WooCommerce subscriptions have _wcpay_transaction_id meta (?)

The code probably lives in the WC_Payments_Subscription_Service. What is notable, that in some places self::is_wcpay_subscription() is used to determine if it is a Stripe Billing subscription, while in other places it is used to decide if the service should handle the subscription update, leading to the inconsistency, described in the issue.