You're running this server on your own LAN, behind a NAT, is that correct? Generally if you're on the same LAN as the server you'll initially see the server in the Internet Servers tab, but it quickly gets filtered onto the LAN tab. That's a local thing only though. Other people will see it show up in their Internet list. I've run into this myself in my own internal testing and as it's the Steam backend that actually provides the server lists to your game client, it's somewhere in there that it decides to classify it as a LAN server from your perspective. People in the office have been able to see my home server just fine in the Internet list though.