New annual authorize.net subscription returns Transaction ID, not Subscription ID

Hi! We use Authorize.net with s2member Pro, and we offer annual subscriptions. Usually our annual subscriptions work fine, but I have a few subscriptions where there is ta problem.

The “Paid Subscr. ID” field in the WordPress user account contains, not the Authorize.net Subscription ID, but instead an Authorize.net Transaction ID. This transaction appears to be the successful first payment of the subscription, except that no subscription has been created in Authorize.net. Normally, I would expect this transaction to link to a subscription, but that is not the case.

Why would this happen?

How can I debug this?

Thank you!
Bill