The true way for update / create a sub field of a group is with this :
update_field('the_group', array('the_element_to_update'=>$value), $post_id)
With this way :update_field('the_group_the_element_to_update', $value, $post_id)
If the field has’nt been created before, it don’t work, because in the database the meta_data _the_group_the_element_to_update is empty and then, you can’t get the value of the field.
`get_field(‘the_group’, $post_id)[‘the_element_to_update’] is empty.
