Why did the battery capacity drop quickly?

Battery Health & Performance 

Your battery health is fine and is undergoing normal usage. iPhone batteries, like all rechargeable batteries, are consumable components that become less effective as they age. You should change out the battery once your Battery Health reaches below 80% which will take over 2 years to reach under normal usage. Until then enjoy your iPhone.

Here is some more reading on an iPhone battery and its performance ➡️ iPhone Battery and Performance - Apple Support

Axel F.  

have you read

About the battery and performance of iPhone 11 and later - Apple Support

and maybe iPhone Battery and Performance - Apple Support

and

Batteries - Maximizing Performance - Apple

people report apple does not change the batteries until they are lower than 80% in health