Any http access to domain issues.jenkins.io should be automatically be redirected to issues.jenkins-ci.org. Http GET access is possible for the domain issues.jenkins.io (e.g. https://issues.jenkins.io/browse/JENKINS-39203 works), but as soon as you try to make changes (create issues, comment, watch, etc.), it does not work anymore. I believe even google has some results for the non-working domain issues.jenkins.io.
Therefore any access to issues.jenkins.io should be automatically redirected to issues.jenkins-ci.org (probably a 301 redirect is best).
Also see the discussion in https://www.google.com/url?q=https://groups.google.com/d/msgid/jenkinsci-dev/f538ef85-f8cb-42fb-b68a-79fe1d239e5a%2540googlegroups.com?utm_medium%3Demail%26utm_source%3Dfooter&source=gmail&ust=1541697039273000&usg=AFQjCNE5PMnND_n9WWV8wxFJw7_kHxBp1A