SSH push mirror: host key verification fails

Summary

Push mirroring from GitLab.com to GitHub.com appears to fail with a host key verification error, even when the workflow to configure mirroring is followed correctly.

Steps to reproduce

1. Create a project on GitLab.com.
2. Create a repository on GitHub.com
3. Configure the GitLab project to push mirror itself to the GitHub project via SSH public key authentication.
4. Once configured, copy the provided SSH public key and upload it to GitHub as a deploy key for the repository.
5. Observe that mirroring always fails with the error message below.

Example Project

• gitlab-gold/ssh-mirror

What is the current bug behavior?

Push mirroring fails with:

13:close stream to gitaly-ruby: rpc error: code = Unknown desc = Gitlab::Git::RepositoryMirroring::RemoteError: Host key verification failed.
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

What is the expected correct behavior?

Push mirroring should be configured successfully.

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com: 13.0.0-pre b99bef61

Workaround

Use https authentication

ZD: https://gitlab.zendesk.com/agent/tickets/155896 (GitLab Internal)