Lost in Saigon
ACARS is not all knowing. If there is un-annuciated fire, ACARS will not know about it and will not report it.
How could it?
That's kind of the whole point of ACARS - to report issues. If a fire affected a single system somewhere, ACARS would have reported it.
See, for example, the 25 automatic messages sent from AF447's ACARS system during its 4 minutes of catastrophic issues