enabling Advanced Configuration from NetworkManager is enough to break reconnect from same client.

Background:
We use HLAPI and Network Manager. Ports are fine and open and checked via http://www.canyouseeme.org/

Issue:
Running two builds on the same machine (one as server/host and other as client) will not connect to each other if HLAPI NetworkManager has "Advanced Configuration" ticked.

Question:
Is this a known issue? a bug? or my admittedly poor experience in networking? I haven't touched any of the Advanced Configuration options, merely ticked it....

enabling Advanced Configuration from NetworkManager is enough to break reconnect from same client.