When checking if a server has a compatible version/is online with the serverlist, the StarMade only tries to connect using TCP. Any UDP server, even if announced, thus will never show up on the list, as they don't respond to TCP requests.
To reproduce:
# set USE_UDP to "true" in the server.cfg and enter the necessary data to announce the server to the serverlist.
# start the server
# open the serverlist in a separate client and search for the server.