ProcessTreeKiller broken in pipeline jobs

After one of the recent updates to pipeline plug-ins the process tree killer always kills all processes even if the BUILD_ID (as described in https://wiki.jenkins.io/display/JENKINS/ProcessTreeKiller) is changed. This is a serious regression because some of our build jobs stopped working.

The following pipeline script can be used to reproduce the problem:

node('master') {
  sh '''BUILD_ID="dont-kill-me bla/blub" sleep 60 &'''
}

The sleep process is killed immediately when the job finishes. The problem only occurs in Pipeline jobs, freestyle jobs still work as expected, i.e. the sleep process continues running for 60 seconds.