Migration of menu links broken

Experiencing weird behaviour with migrated menu items.
I have a migration script that runs after installation which adds a link to the contact form (/contact path).
In the database this appears as menu_tree.url = 'base:contact' instead of menu_tree.route_name = 'contact.site_page'.

After a manual cache clear (in the UI, note: doesn't work with drush cr..), the route_name is magically filled in to 'contact.site_page'.

This migration of menu items used to work in Drupal 8.1 but is broken in 8.2.

The links appear but they aren't indicated as active. In the database route_name is empty (should be "contact.site_page") and route is "base:contact".

The csv source:

id;language;menu_name;link_title;link_path;description;external;weight;expanded;enabled;options
1;en;secondary-navigation;"Contact us";contact;"";0;0;1;1;
2;en;secondary-navigation;"FAQ";faq;"";0;1;1;1;
3;en;footer;"External link";"http://drupal.org/";"";1;0;1;1;
4;en;footer;"Contact us";contact;"";0;1;1;1;

The migrate yaml:

source:
  plugin: csv
  default_lang: true
  path: private://migrate/custom/menu_links_paths.csv
  delimiter: ;
  enclosure: '"'
  escape: "\\"
  header_row_count: 1
  keys:
    - id
  column_names:
    0:
      id: Id
    1:
      language: Language code
    2:
      menu_name: Menu name
    3:
      link_title: Link title
    4:
      link_path: Path
    5:
      description: Description
    6:
      external: External
    7:
      weight: Weight
    8:
      expanded: Expanded
    9:
      enabled: Enabled
    10:
      options: Options
    11:
      route_name: Route name
destination:
  plugin: entity:menu_link_content
  default_bundle: menu_link_content
  no_stub: true
  translations: true
process:
  langcode: language
  title: link_title
  description: description
  menu_name: menu_name
  options_source:
    plugin: explode
    source: options
    delimiter: ,
  'link/uri':
    plugin: link_uri
    source:
      - link_path
  'link/options': options
  route:
    plugin: route
    source:
      - link_path
      - options
  route_name: route_name
  route_parameters: '@route/route_parameters'
  url: '@route/url'
  options: '@route/options'
  external: external
  weight: weight
  expanded: expanded
  enabled: enabled
destination:
  plugin: entity:menu_link_content
  default_bundle: menu_link_content
  no_stub: true
migration_dependencies:
  required:
    - menu