@PA28Viking
"...all three types of transmission could have stopped/been disabled at the same time due to human action or technical malfunction"
I believe the authorities have said/implied that they know that "ACARS was disabled by someone on board" and readers here have suggested that's because it did a tidy "log-off", which itself sends out a transmission, rather than suddenly stopping transmissions. But (I think) you're right in saying that all 3 of comms, xpdr and ACARS could have been disabled at the same time at or around the FIR boundary