iPhone 15 pro max Battery

99% is a healthy battery so it's normal, a battery is a consumable so it can happen, depends on many factors, like use and charging. The worst thing you can do is to let the iPhone go totally flat for multiple times or as a habit and then charge it 100%. But if you were using Optimised charging, and used the iPhone normally I'd have my iPhone tested because it's practically brand new so it shouldn't wear out so quickly

Check the iPhone battery health and usage – Apple Support

This is a real good reading

When to charge your iPhone or iPad - Apple Community

Actually it is normal for a 2 month old iPhone to display a maximum capacity of 98%, so you are doing better than average.

After a year, your battery will be at about 88-90%. It will be time to replace the battery as the phone is around 2 years of age.