On
Spiegel.de (German language), it is claimed that the last transmission received from the aircraft was "All right, good night" or the like (translated by Spiegel into German and by me back into English, so exact words may vary). This was received at the occasion of the handover from Singapore to Ho Chi Minh.
If correct, it does not appear abnormal to me: just a standard salutation upon leaving a frequency without any indication of anything untoward.