JSONPath converts JSON objects to PHP objects from https://www.drupal.org/files/issues/2022-10-28/migrate_plus-json-xpath-filtering-3007709-40.patch

json_to_array.php

<?php
// JSONPath converts JSON objects to PHP objects which cause Migrate to
// fail when using the SubProcess process plugin. This function
// recursively converts those objects to arrays to prevent this issue.
$toArray = function ($x) use (&$toArray) {
  return (is_scalar($x) || is_null($x)) ? $x : array_map($toArray, (array) $x);
};
$field_data = $toArray($field_data);

$this->currentItem[$field_name] = $field_data;