Try this out,
add_filter( 'wp_insert_post_data' , 'modify_post_title' , '50', 2 );
function modify_post_title( $data , $postarr )
{
if($data['post_type'] == '{custom post type}') {
if($data['post_title'] == $postarr['post_title']){
update_post_meta( $data["ID"], 'custom_field', '' );
}
}
return $data;
}
Reference: https://wordpress.org/support/topic/modifying-title-before-saving-custom-post