the update_value filter runs before the value is saved so in your function you are attempting to get a value that does not exist yet. The value is passed to your function in $value
but you aren’t using it, is there a reason for this?
Try the following. You may need to do some additional stuff, not sure. The value stored by acf for dates is YYYYMMDD
unction my_acf_update_value( $value, $post_id, $field ) {
$date = strtotime($value);
$formatted_date = date('d F Y', $date);