Public GitLab.com repositories reported as private/"did not return a default_branch" and needing authentication by GitLab API

Summary

Using composer to get in-development code from a public repository returns a bogus error, "GitLab API seems to not be authenticated as it did not return a default_branch. Your credentials are required to fetch private repository metadata"

This happens on public projects which do have a default branch set.

Steps to reproduce

git clone git@gitlab.com:drutopia/drutopia_sdk.git
cd drutopia_sdk
composer update

(Or 'vagrant up' for an environment with composer already installed.)

What is the current bug behavior?

Failed to download drutopia/drutopia:GitLab API seems to not be authenticated as it did not return a default_branch
Your credentials are required to fetch private repository metadata (git@gitlab.com:drutopia/drutopia.git)
A token will be created and stored in "/home/vagrant/.composer/auth.json", your password will never be stored
To revoke access to this token you can visit gitlab.com/profile/applications

What is the expected correct behavior?

All the gitlab.com hosted public, open source projects that make up Drutopia are installed.

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com