Battery health percentage drop

Probably you are referring to Battery Health. 99% in one year is a superb performance. Health always deteriorates over the year.

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)

I have had mine for 2 years, still shows 99%. Not that I believe it for a second, because I have used it for "hotspot" during two long-distance moves and as a phone in between. At some point it will drop to 90% or lower I believe. Just watch the health indicator for 80%.

The displayed Battery Health is only an estimate. Accuracy improves over time. If you charge frequently throughout the day and never let it discharge below 50% or so, the estimate will not be very accurate.

Your actual battery health after 8 months is probably closer to 97%, your iPhone just may not know it yet. Either number is completely normal and expected. Turn on Optimized Battery Charging, let it fully charge every night, and allow it to optimize to your usage pattern.