Resolution:
TibrvTransport.isValid() tests the validity of a transport i.e. to see whether it has been destroyed or not; it does not check for a valid connection.

If the connection to a remote RVD is lost, the transport will present a _RV.WARN.SYSTEM.RVD.DISCONNECTED advisory message.  This will not invalidate the transport (isValid() will still return TIBRV_TRUE if it was doing so before).  If the RV program manages to re-establish the connection with the RVD the _RV.INFO.SYSTEM.RVD.CONNECTED advisory will be presented.

Your program should create a listener for both these advisory messages, so the relevant behaviour can be processed in the callback functions.