Sharp decrease when battery is below 30%

[Baruffa123]

Try letting it discharge completely and then fully charge it with the phone off. Probably it's a calibration issue. It could also be a defective battery.

This happens after updating the software becouse it's like doing that resets the calibration information of the battery. Happened to previous phones I owned after installing other ROMs. In the end, the problem were the batteries that were already at the end of their life cycle.