There can be many reasons for the Samsung S8 battery drain after the update. And some of them are mentioned below.
1.If your battery drains fast after the first update, then it might be due to the bug which has occurred after the update.
2.You might have been running some background apps that are affecting your mobile performance after the update.
3.The other reason for the battery drain is that your screen brightness is too high. Galaxy S8 battery drain after update if it’s screen brightness is too high.
4.Your charger might have issues or is getting old.
5.You haven’t cleared the S8 system cache for a long time.