Issue #2981877 by jesse.d: Do mailchimp list subscription processing in postSave

https://www.drupal.org/project/mailchimp/issues/2981877

modules/mailchimp_lists/src/Plugin/Field/FieldType/MailchimpListsSubscription.php

@@ -267,8 +267,8 @@ class MailchimpListsSubscription extends FieldItemBase {
   /**
    * {@inheritdoc}
    */
-  public function preSave() {
-    parent::preSave();
+  public function postSave($update) {
+    parent::postSave($update);
 
     $choices = $this->values;