gform_field_value_ breaks on multipage form

I'm using 1.6.12 at the moment. Upgrading to 1.7.x will be a major headache (if this is a bug). So I'm hoping that I am just doing it wrong...

add_filter( 'gform_field_value_first_payment', 'gf_populate_prorata_value', 10 );
function gf_populate_prorata_value( $value ){
  $prorata  = get_prorata();
  return $prorata;
}

On a multi-page form, this field gets populated on page 1, but not if I move it to page 2.

Is this is a bug? If it is, Is there a fix, before the 1.7.x branch?

Thanks.