As per Javadoc:

Expected to be serializable either locally or over Remoting, so an implementation of decorate(OutputStream) cannot assume that JenkinsJVM.isJenkinsJVM(). Any master-side configuration should thus be saved into instance fields when the decorator is constructed.

Improve WorkflowTest to

• use RestartableJenkinsRule to simulate Jenkins restarts with a running build (try SemaphoreStep to pause in the middle)
• use JenkinsRule.createSlave to simulate use of an agent and thus Remoting