I've noticed this issue too. We moved our server to G-Portal and it doesn't show up in the server list so I had to put the IP and port into my favorites using steam server browser. I can see the server and connect just fine but everyone else, including the person renting the server, does the same thing and it just shows up as "not responding" for them. They can still join off of me on steam but can't join the server on their own without me.