GFv1.6.4 Beta 1 stops the assigning uniqie number to entries

I have a field that gives a unique number to entries. It was working fine until I installed v1.6.4 Beta 1.

the code

add_filter("gform_field_value_uuid", "get_unique");

function get_unique(){

    $prefix = "MEFF"; // update the prefix here

    do {
       $unique = mt_rand();
       $unique = substr($unique, 0, 8);
       $unique = $prefix . $unique;
    } while (!check_unique($unique));

    return $unique;
}