RR
if ($entity->getEntityTypeId() == 'taxonomy_term' && $entity->bundle() == 'tags') {
$id = $entity->b_tag_author->value;
$query = Drupal::service('entity.query')
->get('user')
->condition('field_bx_id', $id);
$entity_ids = $query->execute();
$uid = array_pop($entity_ids);
$entity->field_uid->value = $uid;
}
}