Cascading failure scenario
It was reported that no overcharging of the battery as a whole may have occurred. This may have been derived from FDR recording of the bus voltage.
A weight loss of 4kg of (one) of the batteries was also reported. This amount is indicative of substantial loss of electrolytic and insulator material of multiple cells.
All this happened very quickly, perhaps faster then thermal runaway by conduction of thermal energy amongst cells.
In a possible scenario, a single cell short would result in the battery voltage dropping, which would have to be detected and the battery quickly disconnected by a means in order to prevent the bus (or charger) driving remaining good cells into redline above 4.3V, which could result in a cascading increase of cell voltages as more cells reduce voltage and leading to the rapid destruction of all cells.
Thus it would be interesting to know whether some cells are shorted and how the battery is decoupled/protected from tied buses.