Comment by rbrtdrmpc-

I own those and opened them a couple of times to replace the battery and the earpads, no need of further investigation, the battery is placed there and for sure there is a short circuit protection in place by mandatory design, it is also unlikely they got pierced by something since they sit in a large space and the battery itself is rounded, so expansion is took into account, to me it seems that the battery was already compromised from the factory which is very very very bad

edit: no idea if the internal thermistor can fail by any means and it could have messed up the charge/discharge logic