Cannot generate routes that have more than one parameter

Hello!

My bug can be reproduced with the following steps:

1. Create a module with a custom route that has two parameters:

example.routing_example:
  path: '/example-page/{id1}/{id2}'
  defaults:
    _title: 'Routing Example'
    _controller: '\Drupal\<module_name>\Controller\ControllerClass::action'
  requirements:
    _permission: 'access content'

2. Create Custom menu links (menu_link_content) that follow the above rule
3. Enable indexing on the menu where these menu links are added
4. Generate sitemap

On generation we get a 500 error, because the route parameters do not match and id1 is empty.

The snippet of code responsible for this is in Batch.php on line 176

  $route_parameters = array(key($route_parameters) => $route_parameters[key($route_parameters)]);

This forces the route parameters to contain only the last parameter in the route.

Was there a specific reason for forcing this behaviour? Because after unserializing, the $route_parameters array should be in the correct format.

I can upload a patch that resolves this, but I just wanted to make sure that I'm aware of all the implications.

Thank you!