Playing in Singleplayer allows other players to connect to the internal server from the Internet.
It may be important to note that I commonly run the dedicated server to play with friends, and have the appropriate port forwarded. I'm also using the new launcher and run the steam version.
I haven't tried this in a "fresh" install yet as I don't think it would change much. However if you have trouble reproducing this I can look into it further.
There is really not much else I can say about this other than that players are able to connect to the internal server when they shouldn't.