GitHub repository configuration doesn't persist credentials

It seems that credentials provided for source checkout in the GitHub repository configuration aren't persisted.
The plugin we develop depends on the Git plugin as we perform different SCM operations.
When we try to fetch the repository credentials via:

repository.toConfiguration().getString("repository.github.username", "")
repository.toConfiguration().getString("repository.github.password", "")

Empty strings are returned.
Has the API been modified?