I think its the same problem as my problem:
http://support.advancedcustomfields.com/forums/topic/relationship-field-works-after-click-update/
And Bones problem: http://support.advancedcustomfields.com/forums/topic/using-update_field-for-taxonomy-terms/
He found that it worked if he used the field_id instead of field_name when doing a get_field. Maybe that could solve your problem?
