Free trial subscriptions require manual renewal when ordered from the block checkout

[ricardo]

Describe the bug
The issue was initially referenced in this comment.

For some reason, when the order contains a free subscription, the _requires_manual_renewal post meta is set to true, and the payment method is not set for the subscription.

The goal of this issue is to track down the problem and see if we can fix it or we should open another issue in the subscriptions or in the Woo Gutenberg blocks repositories.

To Reproduce

1. Ensure you have WooCommerce Subscriptions and WooCommerce Blocks installed.
2. Create a free trial subscription product.
3. As a shopper, add the free trial subscription to the cart.
4. Go to the blocks checkout page and complete the order.
5. As a merchant, go to WooCommerce > Subscriptions and notice the subscription says "Via Manual Renewal".

Expected behavior
Instead of manual renewal, the payment method should be used.

[ricardo]

The same issue happens in WCPay with the same repro steps.

[ricardo]

I tracked down the issue last week and found out that the blocks API is not setting the payment method metadata if the order doesn't require payment (reference).

This causes free trial subscriptions and possibly other types of off-session charges like pre-orders to require manual payments.

The resolution for this issue needs to be implemented in WooCommerce Blocks.

[ricardo]

Closed in woocommerce/woocommerce-blocks#4854