No token hook substitution for CiviMail cron job under Drupal

Steps to reproduce:
1) Define token hook function in Drupal module, e.g. mymodule_civicrm_tokenValues().
2) Create mailing, using token that is affected by the token hook function.
3) Send test mailing.
4) Confirm token replacement in test mailing.
5) Submit job for cron scheduling.
6) Token not replaced in email submitted via cron jo