rafaelzgz good point, it must be because of the Websockets extension.
It's true that there would be a conflict between the versions of pusher/pusher-php-server required, because flarum/pusher requires ^2.2 and kyrne/websocket requires ^7.0. But the -W shouldn't uninstall packages that are listed in the root composer.json.
However a different Composer feature can remove packages, it's the replace attribute. kyrne/websocket says it replaces flarum/pusher in its composer.json, so this causes Composer to automatically remove it. The Composer output should say that the package was removed when you installed the other one.
You might be able to re-install the two together with some trickery. But do you actually need it if you switched to the Websockets extension? Best to keep only one of them installed.
