The underscore symbol is not recognized as part of the url
The resource can't update the status of the pull request if the name of the repo contains underscore. It gives the following error:
put failed: failed to set status: POST https://api.github.com/repos/Test/test-test_test/statuses/73a21b9b4d39386b85c765d117fc3af98f35a82a: 404 Not Found []
That may be an issue related to GitHub not supporting underscore characters _ in repo names, don't you think?
Anyway, there is no hope that you get any further support here (#246 was 2+y ago).
But recent news is that the Cloud Foundry community is now maintaining a fork of this resource at cloudfoundry-community/github-pr-resource.
On our Concourse installation at Gstack, that one is working fine. Could you give a try and provide feedback there?
(the new resource is a drop-in replacement; you only have to switch telia-ossto cfcommunity in in the resource_types: declaration, i.e. specify repository: cfcommunity/github-pr-resource)