Art-Net 3 Node's are only showing in the list when their merge mode is set to LTP.
If you activate the Unicast option of ArtNet output, MadMapper shows a table of devices on the network. This table is used to know to which IP addresses we should send the packet for each universe. You can fill the table manually, but in a normal situation, it fills itself except if you deactivate "Auto-Poll for devices".
MadMapper knows which devices are on the network thanks to ArtNet Poll mechanism: we broadcast a message each N seconds to ask each device to send us a packet describing himself.
More technically, we except a ArtNet PollReply frame (protocol spec here: https://artisticlicence.com/WebSiteMast ... rt-net.pdf
There is there a field called "GoodOutput" (one per universe the device is listening to), if it's 0 MadMapper ignores the associated universe. I just read the spec again and I'm not sure this test is correct, but since years this hasn't changed and we had no report of polling not working until now. However we'll change it in 3.6.3 to ignore this field. Thanks for your feedback.