By default, Yoast SEO removes `?replytocom` variables from URLs. This can be reversed with the following code.

yoast_seo_replytocom.php

<?php
/********* DO NOT COPY THE PARTS ABOVE THIS LINE *********/
/*
 * Yoast SEO return `?replytocom` variables
 * Credit: Yoast development team
 * Last Tested: Mar 23 2018 using Yoast SEO 7.1 on WordPress 4.9.4
 */
 
 add_filter( 'wpseo_remove_reply_to_com', '__return_false' );