A few clues
1) It is foolish to rely on FR24, the sytem works by collecting data over the internet from amatuer ADSB receivers owned by enthusiasts and joining the dots. There are plenty of these stations in well populated areas but the chances of any reliable receiver data in the middle of an enormous desert are a tad slim. In these circumstances FR24 extrapolates (guesses) by extending the last known trajectory until it recieves a valid report or times out.
It is therefore not possible to derive a rate of decent from the last guessed postion and a single and possibly unreliable following report.
2) "Vanished from radar" does not imply a problem, it just means that the aicraft moved outside of the limited coverage range of a civilian airport radar.
3) Anyone who uses the word "Plunge" is either a tabloid journalist or one of their clueless readers. Best ignored.