Battery health reduced 5% after IOS 15 update

The battery health displayed by iOS is not a real time value. It updates, and sometimes in chunks. Not sure why, but that's kind of how it works. For example, the value might hold at 95% for 8 months, then overnight it's at 91%. That doesn't mean it lost 4% all at once. It means that it's been slowly losing capacity but that's when it got reported.

It's also only an estimate based on the power management observing the battery.

Welcome Samfrancis9,

Thank you for using Apple Support Communities. Based on your post, your Battery Health has reduced after updating your iPhone XR. We'll do our best to assist with this.

It's normal for the Battery Health to decrease over time and usage. To learn more about how these batteries age, please see: iPhone Battery and Performance

To check battery usage: About the battery usage on your iPhone, iPad, and iPod touch

Maximize your battery life with these tips: Batteries - Maximizing Performance

With that said, at 80% Battery Health, Apple can replace the battery with a new one.

While you aren't to that point yet, these will help if you decide to seek service: iPhone Repair and Service

Get prepared for your visit: Get your iPhone, iPad, or iPod touch ready for service

Select your region and search nearby for locations: Find Locations

Due to the current conditions around the world, some stores may have limited hours or appointment times. They also may require specific things to visit them (Such as, scheduling an appointment, wearing a mask, calling to check-in, etc.).

Wishing you well, take care.