Can't change billing details

When trying to use the "change billing details" option in a membership, a there was some problem updating the billing details error message, as shown in the screenshot:

With a step-by-step debugging session, I found that CRM_Core_Payment_Smartdebit::updateSubscriptionBillingInfo isn't properly reading the subscription reference:

In particular, it's receiving the subscription reference in the $params array as ddi_reference but reading it as subscriptionId. The consequences I've found are:

that's causing the call to CRM_Smartdebit_Api::requestUpdate to be done with NULL as a reference,
later, CRM_Smartdebit_Api::requestUpdate is building the SmartDebit API call without a reference:

$url = CRM_Smartdebit_Api::buildUrl($paymentProcessor, 'api/ddi/variable/' . $reference);

The line above should be generating a URL like api/ddi/variable/REF001234 but it's generating something like api/ddi/variable instead.

Environment

CiviCRM 5.35.1 (also happens with 5.36.1)
SmartDebit 1.38
MJWShared 0.9.12

Edited May 14, 2021 by capo