The mailer plugin does not include the changes set in the failure mail when using a pipeline.

By looking at the issue JENKINS-43202 and in the code, it seems to be related to the workflow job not inheriting from AbstractBuild and, if so, explicitly (https://github.com/jenkinsci/mailer-plugin/blob/master/src/main/java/hudson/tasks/MailSender.java#L217-L220) returning an empty set .