Hi Michael,
similar problem here.
Seems like some Server2016 (and 2012) not appearing in ATP.
The operations Manger log shows socket errors.
Winhttp proxy is set but it looks like it's not used. I can see the :80 and :443 packets in out firewalllog, nothing in proxylog.
If i open the ports 80/443 for one of the servers to any external IP, it appears in ATP.
Found nothing about this problem in the internet - searching for many days.
Greetings