The card selected for a manual credit purchase applies to that purchase. It does not replace the payment method on an existing Forecast or scheduled-card agreement. Those settings intentionally have a separate automatic payment method.
Review that method in Auto-top-up settings. Where available, Use another card there opens an agreement update rather than a credit purchase; an overdue agreement needs recovery through the billing portal. If the update completed but future charges still show the old method, contact support with the update time and masked card details, never the full card number.