Composer installs this module despite no dependency is given on it

Problem/Motivation

Today I ran a composer update on a codebase that is using "drupal/tour_ui" as a dependency. That tour_ui module itself seems to have a dependency to core's tour module (declared as "drupal:tour" in the info.yml file, no composer.json is given).

However composer installed this contrib module, and I don't know why it did. Running "composer why drupal/tour" printed the information "There is no installed package depending on "drupal/tour"".

Steps to reproduce

Setup a composer based project, require drupal/tour_ui. Run composer update on it. On my installation it then installed "drupal/tour".

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes