Jenkins github pull request builder picks wrong commit

The problem was the branch specification and refspec. Changing the scm.git section of job to this solved the problem of Jenkins checking out the wrong commit:

scm {
    git {
        remote {
            name("origin")
            url("[email protected]:org/${repo}.git")
            credentials("jenkins-ssh-keyid")
            refspec('+refs/pull/*:refs/remotes/origin/pr/*')
            }
            branch('${ghprbActualCommit}')
        }
    }
}