Hello,
The Server Heartbeat is not a keepalive. Actually it is the opposite. It is an orphan cleanup feature. It detects if the device has shutdown, then closes the session. If your device does not support the mid-session telnet protocol exchange, then it could close sessions down unexpectedly.
Please check the Event Viewer under Windows Applications and Services Logs -> Pragma TelnetServer for an information event that we shut down due to a lack of response to a server heartbeat.