Killing the OpenSSH server doesn't cause the Windows OpenSSH client to die

[Brian Candler]

Try setting

Host *
   ServerAliveInterval 240

in .ssh/config on the client (wherever that is in Windows).  Or pass "-o 
ServerAliveInterval=240" on the ssh client command line.

I've seen this where connections pass through some low grade external 
firewall, which drops state after a certain amount of inactivity without 
sending back RST (and apparently is immune to TCP keepalives too).

You say both boxes are on the same LAN, but it's possible that a 
badly-configured software firewall on the FreeBSD box could be the cause 
- or on Windows itself.