Just because the battery and charger 'check out ok' in investigative tests is no guarantee that redesign will not be mandatory.
Irrespective of the initial cause of malfunction (which may be upstream of the battery/charger or may be due to some overlooked variable) the outcome was a non-extinguishable burning battery. Which implies at least that this system is significantly intolerant to external contingencies.
A corollary is hydrogen in airships: it is lighter and cheaper than helium, but has distinct chemical properties which can on occasion prove disadvantageous...