SOLVED by replacing GitHub Jenkins plugin “branch discovery strategy” - thanks to PR decoration for GitHub no longer working.
“I’m not sure if it could be a problem, but how our jenkins build works is it merges master into the branch and creates a new branch named PR- eg PR-123. I think this is standard jenkins behaviour though, and we don’t build the actual branch during pull request as we don’t care about it. But I guess github wont have the same sha1?”