Well, I misunderstood what you mean, sorry 
I thought Core Tunnel failed to route traffic despite the remote side still alive. The actual situation is the remote side became unresponsive, but Core Tunnel didn't disconnect as expected.
To let Core Tunnel detects if server still alive, we must set ServerAliveCountMax and ServerAliveInterval explicitly:
In next alpha, new tunnels will have 3 for ServerAliveCountMax and 15 for ServerAliveInterval as default values.