Disabling language prefix for route in module
Yes, has to be redirect.module, core doesn't do that. See https://www.drupal.org/node/2854462#comment-12047347, just add the '_disable_route_normalizer' flag to your route and you should be fine.

module_sitemap.routing.yml:

module_sitemap.sitemap:
  path: '/sitemap.xml' 
  defaults: 
    _controller: '\Drupal\module_sitemap\Controller\SitemapController::view'
    _title: 'View Site Map'
    _disable_route_normalizer: 'TRUE'