Good question — here's how it works.
Changing your card on the Pay Now screen only applies to a payment you make right then. It doesn't change the method your automatic payments use going forward. So if you switched the card there but didn't complete a payment, your next scheduled payment still used your default method.
To change the method your automatic payments use from now on, update your default payment method at splitpay.com/account: open Payment Methods and set the method you want as your default.
If you just want to use a specific method for one upcoming payment, open the Split Pay app before your due date, choose Pay Now, pick that method, and complete the payment.
Still seeing the wrong method after updating your default? Contact our support team and we'll take a look.