iphone 12 mini battry drain in night

Well, the company is wrong about charging.

I understand how how to deal with batteries, and have been using electronics for quite a while. As far as drain for no reason, the only drain observed in your device is from its use. That is what those blue bars display. The 4 hours where there was no bars visible that you boxed in red do not display losing half your battery. I'm afraid you are confused about how to read that graph. During that time, you may have lost 2% battery, which as long as the device is powered on would be not only nominal, it would be normal.

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)

The very slight drop in battery from the time that you have circled is normal. A battery is still going to drop a percentage or 2 over the course of several hours. I'm not even sure I understand why you do not charge the phone overnight to 100%, and especially since that allows you to do an iCloud backup of the device at the same time.