Swissair 111 had a fire on board which disrupted the systems. We don't know what happened on MH370 but the speculation is that circuit breakers were pulled.
Consider your own PC. If you shut it down normally the event is logged, that is quite different from terminating the power by pulling the plug out (or burning the cables). In that case, you won't get a shutdown log but, depending on how the system is configured, you may get a message on reboot saying that the system was not cleanly shutdown.