Please forgive me, below is a conversation I've had with a group of Server Hosters.
This is all linked to headless client attempting to connect to a DS on the same machine.
Steam... locks the local DS session once client connects, 'connection failed' shown to external clients. Linked to: http://feedback.arma3.com/view.php?id=13857
Only ever got HC to work once when steam was fully logged in which fetches a popup "would you like to launch a3" which is not ideal when hosting servers as the process is not started till acceptance on the GUI.
Also noticed that binding a dedicated server to a IP '-ip=', then trying to connect by loopback fails also, again 'connection failed' when trying to join the game (headless client arma3.exe -client) which is in most ways flawed. This could be caused by having more than one active NIC on the server - again I do not know.
Confirmed... steam is in online mode when HC connects and holds its connection to the DS without being kicked in 60-120seconds. However it seems to lockout the Dedicated server at this time prevents external clients from connecting.
If you put the HC Steam account to 'offine' mode then join it gets kicked, however other external clients can connect.
I should also point out that both the DS and HC have totally different accounts. The only familiarity between them is that they're on the same machine and the same NIC.
DS is installed by SteamCMD, and HC by the actual client due to that it does not seem to authenticate through steamcmd correctly and thus does not launch.