@LewisJones were you in sports mode at the time ? if so thats put quite a current draw on the battery ,especially if you were videoing ,and had the obstacle avoidance active as well
another issue with the
mini 3 pro, is it gets quite hot anyway during flight ,and in sunny conditions just hovering it can overheat
DJI have packed so much into such a small package, and therefor no internal fan, to save weight ,it only has the limited airflow as it is moving to help dissipate heat build up
none of this excuses the fact that the battery somehow went into a thermal runaway ,hope you can get a replacement ,sorry for your drone loss