There appears to be an issue with the github-branch-source plugin where if you put a specification on a credential's domain, the credential will never be selected to authenticate when creating a GitHub Status. This appears to be because the apiUrl is not being passed when looking up the credential.

The project build log will display:

Could not update commit status. Message: {"message":"Must authenticate to access this API.","documentation_url":"https://developer.github.com/enterprise/2.9/v3"}

I've got a patch incoming and have tested it out for a hostname specification and was able to successfully have it fail before and succeed after the patch. I also reverted to 2.2.2 and the failure returned.