Subscriptions API endpoint not returning subscriptions

I have successfully set up the product and checkout (in Test mode) on our Wordpress, but the subscriptions API does not reflect that. The subscription is visible in the dashboard and the orders + customers API endpoints return the relevant customer + order info, but for some reason does not seem like it registered the purchase as a subscription.

For instance, the customers API returns this bit of the payload under statistics:

{
...
statistics: {
  ordersAmount: 1
  ordersCount: 1
  subscriptionsCount: 0
}
...

Any help here would be appreciated, thanks!

EDIT: this may be relevant: the paymentSchedule was configured like this:

paymentSchedule: {
  interval: 0
  intervalCount: 1
  startsOn: "2021-11-18T00:00:00Z"
  trialPeriodInDays: null
}

I’m wondering if interval: 0 breaks the subscription behavior? I’ll try another purchase with a different interval.

EDIT 2:

I also don’t see any subscription webhook coming in, only order.completed webhook.

EDIT 3:

Sorry, please disregard Edit 2, I see this webhook only comes in after subscription payments begin happening.