When using slave.jar, build nodes publish information about currently running builds via the Jenkins API. I'm not super well-versed in the API internals, but as far as I can tell, this involves publishing extra information for each executor. Otherwise, Jenkins replaces this with a PlaceholderTask, which doesn't contain any information about what is actually building, and the API consumer can only see if the executor is idle or not.

Effectively, this means that API calls such as [{{get_running_builds}} in the python-jenkins API|]https://python-jenkins.readthedocs.io/en/latest/api.html#jenkins.Jenkins.get_running_builds return an empty list, which is not terribly useful.