Hi,
All my gems are up to date. My frontend packages on the other hand very outdated, however it doesn't matter in the matter of this issue.
The CLI command
bundle exec rake ember:compile
builds by default assets into tmp/ember-cli/apps/frontend/assets and there is some magic that makes
GET http://localhost:3000/assets/vendor.css
possible.
However, starting with Sprockets 4.2.0, these GET http://localhost:3000/assets/vendor.css makes 404.
Just though I would share in case somebody else hits the same bug.