It was clarified at the press conference today, that they do not know when the ACARS was turn off/stopped. They only know that the last transmission was at 17.07 and that the next transmission expected 17.37 never happened.
Which presumably means that, contrary to supposition, no tidy "system logoff" message was received, and that in fact there is nothing to support the hypothesis now rampant that the crew or some other person deliberately disabled the transmission of ACARS data, and reopens the possibility that it was lost through some failure event or other?