iPhone 8Plus battery drains even in idle. From 60% to 0% during night

Protect & Prolong the life of your iPhone's Battery: Batteries - Maximising Performance - Apple (IN)

Try some of these options too.

1. Avoid Screen Mirroring
2. Put on Low Power Mode
3. Put Auto-Lock on (1 min):- with low power mode it will be 30 Sec
4. Put Auto Answer Off
5. Disable Push notifications for apps you do not use often
6. Keep your iPhone Facedown to prevent notifications to light up the screen
7. Enable Auto-Brightness:- Settings > General > Accessibility > Display Accommodations > Auto-Brightness [Enable]
8. Manually lower brightness from Control Center
9. If you Want to watch continuous Video. Stream it on TV or a Monitor
10. Don't force close apps; Apps that are not on the frontend screen are paused by the system to save energy. Apps require more energy to re-launch. Apps that are open in the background use large memory is a myth and quitting them is counterproductive
11. Location services to be allowed only while using the app
12. Set your iPhone to fetch email and calendar data from the internet less frequently
13. Turning off Background App Refresh 
14. Charge before it drops below 20-30%
15. Try connecting to trusted Wifi wherever available instead of Cellular
16. Enable Airplane mode in areas with bad cellular and wifi coverage. Your phone keeps trying for reconnection with the missing/weak network. This uses a lot of energy
17. Switch off Wifi & Bluetooth while in Roaming or Not in Range
18. Enable "Wifi Assist" : Settings > Cellular or Settings > Mobile Data > Wi-Fi Assist [Enable]
19. Periodically Check the Battery Health (Settings > Battery > Battery Health)

rajaboopathyg wrote:

Apple, please fix this immediately.

You are addressing this to another user and not to Apple in this forum.

Apple Support Community Forum

This is Apple Support Community forum, primarily for a user to user interaction for information & idea sharing between users, if need be, extend support on technical know-how in handling Apple products from a more experienced fellow user, This is not the right forum to discuss with Apple Inc.

rajaboopathyg wrote: "...changed by iPhone battery from authorized apple service center..."

That can be verified here:

Repair Provider Verification - Official Apple Support

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Besides everything else that has been said, battery issues are normal just after updating.

Also see here - The last update killed my battery! - Apple Community