It sounds like the same thing happened a few years ago off California. ATC was relying on transponder data and the data was wrong. That aircraft went in with all passengers except there was another aircraft in the vicinity that saw the whole thing.
They were getting overspeed and stall (too slow) warnings plus their altitude was showing good but they were actually in a dive. They ended up stalling the aircraft because they thought they were going too fast and crashed.
Chinese aircraft have seen “Suspicious objects” in the ocean..if this is parts of the plane at least let their be answers to this mystery