Decreasing battery health

Curious to know if you updated to iOS 14.5, when 14.5 was released? If you haven't updated, you need to. iOS 14.5 introduced software to recalibrate the Battery Health for iPhone 11 exclusively to be more accurate.

See this --> About recalibration of battery health reporting in iOS 14.5 - Apple Support

As I mentioned, the battery health could up, go down or remain the same after the battery health recalibration. When the battery health reduces to 80% or less, it will be time to have Apple replace the battery in your phone.

A few thoughts for you:

1. Never. Never. Ever let the battery deplete to 0% - it can reduce battery health
2. Don't let the charge on your phone go below 20%
3. Make sure in Settings > Battery > You have Optimized Charging turned on
4. Do plug your phone in to charge at night when you go to bed and leave it plugged in all night

Take care

It can happen.

If it drops below 80% inside 12 months warranty period put in a warranty claim otherwise have Apple change the battery at 80% at your expense.