This issue manifested itself since October 15th.
When a build is triggered and it runs on multiple slaves parallely, you normally get an output for each line corresponding to the variant its running. Since a Jenkins update on October 15th, this has not been the case. This is making it harder for us to debug the issues on the variants/slaves, as we don't know on which variant the issue is being manifested.
Although unrelated, this issue was manifested together with another bug which doesn't allow an anonymous user to view jobs, even if logged in as a non-admin. Only users with admin privileges are allowed to view the logs. That issue has been found on your backlog. https://issues.jenkins-ci.org/browse/JENKINS-54031