Hi Sontaya,
1. We review the FA and found no abnormality with capacitors manufacturing. The condition of external cathode layers appears to be correct.
2. The burnt area represent the point of dielectric breakdown. The dielectric is a film of tantalum pentoxide formed on the overall surface of the tantalum particles by means of an electrochemical oxidation. Impurities included in the dielectric. Oxide crystal spots already included in the dielectric. The combination of the electrical over stress (causing to thermal shock) and thermal-chemical reaction damages the tantalum oxide layer.
The dielectric breakdown can occur any were on the Ta pellet surface with no mean of anode or cathode area.
Keeping de-rating guide lines and avoiding spikes in the circuit will last long the Ta capacitor.