add_filter('acf/load_value/key=field_12345', 'load_my_field_12345', 20, 3);
function load_my_field_12345($value, $post_id, $field) {
// check for empty value
if ($value !== NULL) {
// you may need to check for something else specific to this field type
// but not being empty indicates a value was already set for this post
// so bail
return $value;
}
$value = "whatever value is appropriate for this field type":
return $value;
}
