Hi,
Please pass the parameters to the helper function in the correct order.
add_action( 'rwmb_district_tax_rate_after_save_field', function ( $null, $field, $new, $old, $object_id ) {
rwmb_set_meta( $object_id, 'district_proposed_rate', $new );
}, 10, 5 );
and the field district_proposed_rate
should be a text field.