After doing some digging we found on our instance (Odoo 17) by default the tokenization wasn't set for ACH payments. It was however for cards. This led to the issue of a customer paying their invoice to a subscription, and only the card was available because the ACH Direct Debit wasn't configured for recurring payment. In order to resolve this, navigate to Sales > Configuration > Payment Providers > Authorize.net > Enable Payment Methods. Ensure the Tokenization Supported is selected at which point you should be able to take ACH payments for subscription products.