Issue #3408643 by gooddev, jsacksick: Reset payment_method when changing payment_gateway.

src/Plugin/Field/FieldType/PaymentInstrumentItemList.php

@@ -132,6 +132,15 @@ final class PaymentInstrumentItemList extends FieldItemList {
       throw new UnprocessableEntityHttpException('Payment gateway does not exist.');
     }
 
+    $payment_method = $order->get('payment_method')->entity;
+    $payment_gateway_from_method = $payment_method?->getPaymentGateway();
+    // If the order references a payment method that belongs to a different
+    // payment gateway, clear the order payment method.
+    if ($values['payment_gateway_id'] &&
+      $payment_gateway_from_method?->id() !== $values['payment_gateway_id']){
+      $order->set('payment_method', NULL);
+    }
+
     $payment_gateway_plugin = $payment_gateway->getPlugin();
     if ($values['payment_method_id'] !== NULL) {
       $payment_method = $this->entityRepository->loadEntityByUuid('commerce_payment_method', $values['payment_method_id']);