iPhone battery health

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.  

Batteries degrade with use ... nothing to stop it. If your phone is about 4 months old, that is normal. Charge it at night so it learns your usage patterns, keep "optimize" on, replace battery at 80%.

That can happen. For whatever reason, the battery health isn't a real time reading and doesn't seem to be reevaluated on a daily basis. For example, it could be something like 100% for the first 6 months of a battery, it drops to 99%, then it drops to 95% after another 3 months. It seems strange, but often it seems to delay updates, and then it seems like it's just losing 3-4% at one time when the battery degradation has been accumulating for some time before the update.

Save some sort of catastrophic damage, batteries don't really lose 4% battery capacity overnight.