That is a concerning failure rate.
In production we use these batteries on a daily basis with no issues. The only time we see failures is when the battery has seen a lot of use and eventually the power leads can break off from their soldered connection to the tabs on the cells. This happens when too much force is applied to the battery power cable typically when unplugging the battery from the drone.
It is imperative that whenever unplugging a battery from anything, all your pulling force must be applied through the XT30 connection. If force is applied anywhere else, there is a risk of damaging the power lead connections to the battery
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And as a side note/reminder: please do not spin the motors when using the wall power supply, doing so will damage the ESC