Override save to not store the username field

system/src/Grav/Common/User/User.php

@@ -91,6 +91,20 @@ class User extends Data
         return (bool) $result;
     }
 
+    /**
+     * Save user without the username
+     */
+    public function save()
+    {
+        $file = $this->file();
+        if ($file) {
+            $username = $this->get('username');
+            unset($this->username);
+            $file->save($this->items);
+            $this->set('username', $username);
+        }
+    }
+
     /**
      * Checks user authorization to the action.
      *