Stripe: default payment method not retrievable via API

With the Stripe API, I’m trying to retrieve the default payment method of a customer via first via the subscription, then via the customer profile. Like so:

try {
            $subscription = StripeSubscription::retrieve($subscriptionId);
            $defaultPaymentMethodId = $subscription->default_payment_method;

            if (empty($defaultPaymentMethodId)) {
                $customer = StripeCustomer::retrieve($customerId);
                $defaultPaymentMethodId = $customer->invoice_settings->default_payment_method;
            }

            if (!empty($defaultPaymentMethodId)) {
                $paymentMethod = StripePaymentMethod::retrieve($defaultPaymentMethodId);
                $last4 = $paymentMethod->card->last4;
                $exp_year = $paymentMethod->card->exp_year;
                $exp_month = sprintf("%02d", $paymentMethod->card->exp_month);
                $brand = ucwords($paymentMethod->card->brand);
                $has_payment_method = true; // Set to true if a payment method is found
            } else {
                $last4 = $exp_year = $exp_month = $brand = null; // Set these to null to avoid potential issues
            }
        } catch (Exception $e) {
            logError('Stripe API error trying to get payment method details: ' . $e->getMessage());
        }

In both cases, I’m getting a ‘null’ result, even though I can see in the Stripe dashboard that the customer has, in fact, a default payment method set: (I only accept cards, so it has to be a card)

I have tried all sort of trouble shooting. API works fine for other work, I can retrieve all the other info of customers, just not the default payment method.

Any ideas what else I could try?