glitchy
This is engine-status VHF ACARS we're talking about, right? Satellite monitoring of VHF traffic is well within the realm of the possible, but the job is finding the relevant ACARS blocks in what's likely a mountain of noisy signals intelligence data, not to mention getting it approved for release.
This scenario is the one that would seem to make the most sense for why it his taken so long to discover any pings past the loss of the transponder.