Sounds like inadequate current delivery on continuous high power mode under load had dropped the voltage enough to make it appear the battery was almost empty, and this can trigger a cutout. A full recharge should leave everything ok again, so see how it performs after that.
Also do a capacity check after that recharging. To do that, hold in the level check button for at least ten seconds and the number of LEDs indicates the capacity of the cells. If it's less that the full five LEDs, post back for further advice.
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