Different Result in Code Than API Explorer
At the point hook_post is called the timeline (activities) hasn't been created and the association with the client (CaseContact object) hasn't either.

See https://github.com/civicrm/civicrm-core/blob/master/CRM/Case/Form/Case.php and https://github.com/civicrm/civicrm-core/blob/master/CRM/Case/Form/Activity/OpenCase.php