Yes, this looks like a BI Master Server issue indeed.
We've had a similar issue with an other ARMA 3 Alpha game server in the past, we've never heard back from it, which usually means the issue had been resolved.
Since when did the server stop showing up in the Master Server list?
We could wait a while, to see if it fixes itself on BI's end.
Or we could try to change the port number. If that doesn't do the trick, change the IP as well. (though not sure if that is desirable, as others might have added the current ip:port to their favs-list?)