From a brief glance at https://github.com/jenkinsci/concurrent-step-plugin I would say that it is designed incorrectly (confuses “native” Java threads with “virtual” CPS VM threads) and should not be used. Most or all of its steps probably could be reimplemented correctly while using the same Pipeline script interface.
This plugin came up in discussion on Jenkins JIRA:
https://issues.jenkins.io/browse/JENKINS-44085
Jesse Glick made this comment:
https://issues.jenkins.io/browse/JENKINS-44085?focusedCommentId=405935&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-405935