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:
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.