[SOLVED] Alias domain gets redirected by Wordpress or ends in 404

We have a domain alias.com , which is a CNAME to domain.com . When we navigate to it (it's a Wordpress site) we get unexpected redirects. E.g. https://alias.com/ => redirected to https://originaldomain.com/en/

Even with malformed paths if it's a different page than the home: e.g. https://alias.com/contact => redirected to https://alias.comcontact (the slash from /contact got removed magically!).

If we visit a page with the language in the path , like /en/contact , it works only with the original domain. With the alias domain it gives 404 error (by Wordpress not the web server , since we see a WP page with our theme showing the 404 error , not the default web server's 404 page) .

We verified the redirects are by WordPress not the web server , since the response headers have a "X-Redirect-By: Wordpress".

The funny thing is: the www. works well. i.e. https://www.originaldomain.com/contact correctly loads the page. It's a CNAME like alias.com , so we expected alias.com to work the same.

Now the question is if it's the core WP to blame , or some plugin.

The most likely plugin to blame between the ones we have is TranslatePress , which we really need (cannot disable).

For now we just want to investigate (with custom hooks in functions.php , logs or something) , to ascertain that WP core is to blame , not a plugin like TranslatePress.

How would we do that?

We have also this in wpconfig

define('WP_HOME', 'https://' . $_SERVER['HTTP_HOST']);
define('WP_SITEURL', 'https://' . $_SERVER['HTTP_HOST']);

and this is our .htaccess

# BEGIN WordPress
# The directives (lines) between "BEGIN WordPress" and "END WordPress" are
# dynamically generated, and should only be modified via WordPress filters.
# Any changes to the directives between these markers will be overwritten.
Options -Indexes

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

# BEGIN cPanel-generated php ini directives, do not edit
# Manual editing of this file may result in unexpected behavior.
# To make changes to this file, use the cPanel MultiPHP INI Editor (Home >> Software >> MultiPHP INI Editor)
# For more information, read our documentation (https://go.cpanel.net/EA4ModifyINI)
<IfModule php8_module>
   php_flag display_errors Off
   php_value max_execution_time 300
   php_value max_input_time 60
   php_value max_input_vars 3000
   php_value memory_limit 512M
   php_value post_max_size 516M
   php_value session.gc_maxlifetime 1440
   php_value session.save_path "/var/cpanel/php/sessions/ea-php82"
   php_value upload_max_filesize 512M
   php_flag zlib.output_compression Off
</IfModule>
<IfModule lsapi_module>
   php_flag display_errors Off
   php_value max_execution_time 300
   php_value max_input_time 60
   php_value max_input_vars 3000
   php_value memory_limit 512M
   php_value post_max_size 516M
   php_value session.gc_maxlifetime 1440
   php_value session.save_path "/var/cpanel/php/sessions/ea-php82"
   php_value upload_max_filesize 512M
   php_flag zlib.output_compression Off
</IfModule>
# END cPanel-generated php ini directives, do not edit

# php -- BEGIN cPanel-generated handler, do not edit
# Set the “ea-php82” package as the default “PHP” programming language.
<IfModule mime_module>
  AddHandler application/x-httpd-ea-php82___lsphp .php .php8 .phtml
</IfModule>
# php -- END cPanel-generated handler, do not edit

We have this hook for debugging , in functions.php

add_action('init', function() {
  if (is_admin()) return;
  header('X-DEBUG-HOST: ' . $_SERVER['HTTP_HOST']);
  header('X-DEBUG-HOME: ' . get_home_url());
  header('X-DEBUG-SITEURL: ' . get_site_url());
});

When we navigate to https://alias.com , we get these debug headers

301 Moved Permanently
...
Location: https://originaldomain.com/en/
x-debug-home: https://originaldomain.com/en
x-debug-host: alias.com
x-debug-siteurl: https://originaldomain.com
x-redirect-by: WordPress

then after that redirect the headers are

200 OK
...
x-debug-home: https://originaldomain.com/en
x-debug-host: originaldomain.com
x-debug-siteurl: https://originaldomain.com
x-redirect-by: WordPress

When we navigate to https://alias.com/contact , we get these debug headers

301 Moved Permanently
...
Location: https://alias.com/contact/
x-debug-home: https://alias.com/
x-debug-host: alias.com
x-debug-siteurl: https://alias.com/
x-redirect-by: WordPress

then after that redirect the headers are

301 Moved Permanently
...
Location: https://alias.comcontact/
x-debug-home: https://alias.com
x-debug-host: alias.com
x-debug-siteurl: https://alias.com
x-redirect-by: WordPress

So at the second redirect , somehow the homeurl and siteurl lost a slash.

Thanks