The minor upgrade guide at https://symfony.com/doc/current/setup/upgrade_minor.html#update-the-symfony-library-via-composer states:

Your composer.json file should already be configured to allow your Symfony packages to be upgraded to minor versions.

But Symfony installs use 4.2.* style constraints so this is incorrect and a composer update will only update patch releases, not minor versions. The documentation should be updated to reflect this.

After discussing with @nicolas-grekas on Slack, he asked me to open an issue.