WordPress network site with different home url and site adress with reverse proxy redirects trailing slash to site address instead of home url.

redirect-canonical.php

<?php
/**
 * Plugin name: Redirect canonical to home URL for trailing slash.
 * Description: Fix for WordPress network site with different homeurl and site address with reverse proxy, which redirects non-trailing slash to site address instead of home url.
 * Version: 1.0
 * Author: Utkarsh
 * Author URI: https://utkarshpatel.com
 * License: GPLv2 or later
 * License URI: https://www.gnu.org/licenses/gpl-2.0.html
 */

namespace Utkarsh\RedirectCanonical;

if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

/**
 * Redirect canonical requests to the correct URL based on home_url.
 *
 * This is a temporary solution to fix the issue where host and site domain are different.
 * We override `HTTP_HOST` with home url from wp-config.php and test it on staging first.
 *
 * @param string $redirect_url The redirect URL.
 *
 * @return string
 */
function redirect_to_host( $redirect_url ) {
	$home_host     = parse_url( home_url(), PHP_URL_HOST );
	$details       = get_blog_details( get_current_blog_id() );
	$site_host     = $details->domain;
	$redirect_host = parse_url( $redirect_url, PHP_URL_HOST );
	// If redirecting url host is blog address and home host is NOT same as site url then replace host with homeurl.
	if ( $site_host === $redirect_host && $home_host !== $site_host ) {
		$redirect_url = str_replace( $site_host, $home_host, $redirect_url );
	}

	return $redirect_url;
}

add_filter( 'redirect_canonical', __NAMESPACE__ . '\\redirect_to_host' );