A monitored port had been configured in APM for a specific application monitoring. However ports auto-discovery could not detect the port and hit search did not get any related hits. Tcpdump did show traffic for the interested IP and port.
It turned out that the port was configured as HTTPS with a SSL key but on the port it was actually running HTTP instead.
APMDATA-1969: port discovery does show the port when it was wrongly configured as https