iterate in $data until fields for empty relation are over

src/PersistentData/PersistentDataManager.php

@@ -278,7 +278,10 @@ class PersistentDataManager implements IPersistentDataManager
                     $method = 'set' . str_replace('_', '', ucwords($relation, '_'));
                     $model->$method($relationModel);
                 } else {
+                    while (substr($key, 0, strlen($relation . '__')) === $relation . '__') {
                         next($data);
+                        $key = key($data);
+                    }
                 }
 
                 next($relations);