my_ppp_preview_link.php

<?php

add_filter( 'ppp_preview_link', 'my_ppp_preview_link', 10, 2 );
function my_ppp_preview_link( $link, $post_id ) {
	$found = preg_match( '#_ppp=([0-9A-F]{10})#i', $link, $matches );

	if ( ! $found ) {
		return $link;
	}

	$args = array(
		'preview' => '1',
		'_ppp'    => $matches[1],
	);

	list( $permalink, $postname ) = get_sample_permalink( $post_id );
	$raw_permalink = str_replace( '%postname%', $postname, $permalink );

	return add_query_arg( $args, $raw_permalink );
}