My reply might be late but,
Ive had a similar issue earlier so i used the woocommerce_default_address_fields
instead.
add_filter( 'woocommerce_default_address_fields' ,'customize_checkout_fields' );
function customize_checkout_fields( $address_fields ) {
// Customizing city and state labels
$address_fields['city']['label'] = __('Just a City');
$address_fields['state']['label'] = __('Just a State');
return $address_fields;
}
Hope this works.