Power consumption increases after a system update
Your phone undergoes a series of optimisation procedures in the background after a system update has been completed, which increases the battery drain speed. For example, some apps will need to be readapted: Gallery will be smartly reorganised, and files will be scanned again to detect security risks.
These procedures will be completed within five hours if the phone is fully charged and idle, or within three days under normal use. This optimisation process may take longer if the phone is used frequently during this period. Once the optimisation is completed, the standby time will return to normal.