Hi guys.
Sorry for the late reply.
It is no longer possible to hook into the ‘acf/save_post’ action (with a priority of less than 10) before ACF has saved it’s data.
Instead, you can hook into the validation which is always run before saving. This can be done on a field by field level with the filter acf/validate_field (http://www.advancedcustomfields.com/resources/acf-validate_value/) or the action 'acf/validate_save_post' (no docs yet, but found in core/input.php on line 430)
Also, $_POST['fields'] has changed to a safer $_POST['acf']
Sorry for the inconvenience, and please be sure to let me know if the logic you are trying to accomplish is not possible with these alternatives.
