I've had the same thing happen to my Galaxy phone a couple of times. The battery charge drops from 100% to 21% in an instant, and then every few seconds drops another percent and within a couple of minutes it would be dead. Even hooking it up to a charger didn't do anything. I reset the phone, and hooked it up to the charger again, and it charged back up to 100% without issues. I think it may be an issue with the electronics in the Li-ion battery. Probably also the reason some Li-ion batteries can catch fire for no apparent reason. I don't think it's an issue with GoPro.
I've had the same thing happen to my Galaxy phone a couple of times. The battery charge drops from 100% to 21% in an instant, and then every few seconds drops another percent and within a couple of minutes it would be dead. Even hooking it up to a charger didn't do anything.
I reset the phone, and hooked it up to the charger again, and it charged back up to 100% without issues.
I think it may be an issue with the electronics in the Li-ion battery. Probably also the reason some Li-ion batteries can catch fire for no apparent reason. I don't think it's an issue with GoPro.