The plugin settings are not persistent. I configure the plugin with server description, ip and port etc. I select TCP as protocol.
I do some tests, run some jobs, send some metrics, all OK.
I go to "Jenkins Configure System" and configure some other settings (not of the plugin) and then I see that the protocol configuration of the plugin is set to UDP.
The bug is maybe the the UI does not set the default selection for the protocol to the currently configured and stored value, so it gets changed to UDP once you click "Save" or "Apply", even if it was TCP before.