When the agent remoting process on a kafka node dies (or is manually killed), Jenkins master does not show that on the nodes page
Only once a job is trying to run in that agent, it will get marked as offline.
To repro this, connect an agent using kafka, then kill the process in the node and check the nodes page (http://jenkins:8081/computer/), you'll see the node is still online.
Seems like a heartbeat/keepalive mechanism will be needed, but not sure if this is possible. with the current architecture.