Aliases stop working after composer install
Drush has undergone some change in behavior to simplify the implementation and increase maintainability ahead of the stable release.


Rename [site].alias.yml aliases to [site].site.yml
Spit apart [sites].aliases.yml (no longer supported) into multiple [site].site.yml files
Run drush init if you would like to maintain global alias files.


If $ROOT/../drush/sites is not working, perhaps you do not have a Composer-managed site. The actual supported locations for aliases is __DRUPAL_ROOT__/drush/sites and __PROJECT_ROOT__/drush/sites. The project root is where the composer.json file is located, which is usually at $ROOT/... It might be clearer if the documentation were updated to describe the supported locations in terms of the Drupal root and Project root, as this is now the basis for finding these directories.