Can you create a small example application that allows to reproduce your issue?

I've looked at the code and think i understand what is happening here

In the YamlFileLoader, the

The following line is the key to the problem

$this->addHost($routes, $config['host']);

This is called on "$routes", which is not the original $collection of the method, but a temporary one, containing the route that was just added to it.

See how createLocalizedRoute works in this specific case ($routes is a fresh instance, not the $collection passed in argument)

        $routes->addDefaults($defaults);
        $routes->addRequirements($requirements);
        $routes->addOptions($options);
        $routes->setSchemes($config['schemes'] ?? []);
        $routes->setMethods($config['methods'] ?? []);
        $routes->setCondition($config['condition'] ?? null);

Theses methods work as expected because they are all implemented to apply the data to all its children

But here is the source of the troubles

        if (isset($config['host'])) {
            $this->addHost($routes, $config['host']);
        }

Because addHost either apply the data to its children or ... removes a route from the collection and adds new ones.

So in this specific scenario, the $routes variable will contain the correct routes in the end, but not the $collection on which parseRoute is working.

Now, not sure what is the best way to fix this.. maybe try to remove these "temporary collections" ?

@xabbuh

On a new symfony app

{# config/routes.yaml #}
robots_txt:
    path: '/robots.txt'
    defaults:
        _controller: Symfony\Bundle\FrameworkBundle\Controller\TemplateController::templateAction
        _template: 'robots.txt.twig'
    host:
        en: 'www.example.com'
        nl: 'www.example.nl'
        de: 'www.example.de'

php bin/console debug:router

Expected

 --------------- -------- -------- ----------------- -------------- 
  Name           Method   Scheme   Host              Path          
 --------------- -------- -------- ----------------- -------------- 
  robots_txt.en   ANY      ANY      www.example.com   /robots.txt  
  robots_txt.nl   ANY      ANY      www.example.nl    /robots.txt  
  robots_txt.de   ANY      ANY      www.example.de    /robots.txt  
 --------------- -------- -------- ----------------- -------------- 

Results

 ------------ -------- -------- ------ -------------
  Name         Method   Scheme   Host   Path
 ------------ -------- -------- ------ -------------
  robots_txt   ANY      ANY      ANY    /robots.txt
 ------------ -------- -------- ------ -------------