Sounds like Wordpress doesn’t know how to use X-Forwarded-Host.
You might need to override some env under php_fastcgi to override SERVER_NAME and/or HTTP_HOST I think (which are the ones which have the request’s Host header in them)
Or maybe you can add something like this in your wp-config.php, idk:
if (isset($_SERVER['HTTP_X_FORWARDED_HOST'])) {
$_SERVER['HTTP_HOST'] = $_SERVER['HTTP_X_FORWARDED_HOST'];
}