I found this while filing JENKINS-37809. It seems like WorkUnitContext objects created by Pipeline steps do not have their parameters storied in the .actions field. The plugin works by calling getCurrentWorkUnit() on a node's Executor, uses that WorkUnit's .context to get the WorkUnitContext object, then gets its parameters by referencing the .actions field. That field is populated for builds running on executors for Freestyle jobs, but does not for Pipeline jobs.
I don't really know much Java or understand the internal workings of Jenkins, so I'm afraid I can't supply much more detail than that. I put as many relevant details as I could in JENKINS-37809 and decided to file a bug with the main pipeline component, as it seems like that is where the underlying issue exists.