Update_field() has the same return as the WP function update_postmeta().
When using update_field() the way you are using it you must tell ACF the post ID to be updated
update_field('field_key',$out,$post_id);
If it is returning false then either the value was not updated or the value has not changed. To check if the value is updated look in the _postmeta table in the database for the value of the field name on the post_id being updated.
