Why does my iPhone XS stops charging at 90%?

First go to Settings > Battery > Battery Health and Charging. Is the percent below 80%? If so, you can see abnormal behaviors with the battery as it is considered significantly degraded and can’t work as hard to fully charge. If so, you need the battery replaced at an Apple Store or Apple Authorized Service Provider.

iPhone Battery Replacement - Apple Support

If that isn’t it, it’s also possible your iPhone is pausing charging due to overheating or undercooling. This is normal and your iPhone will normally pause charging to regulate its temperature as the battery being outside the normal operating temperatures isn’t good for the battery.

If a Charging On Hold notification appears on your iPhone or iPad - Apple Support

Siver259 (Author)

Perhaps due to the age and use patterns of the device, evidence

of their time in use appears in such an indelible path; some time

in the next year or two, batteries may need replacement.

You may consider scheduling an appointment with an AASP

or Apple Store genius, for diagnostic or condition checkup.

As a fellow user volunteer here; with a few devices and some Macs

I realize, these have limited spans of usefulness; until replacement.

I have the same issue with my iPhone 16 pro. Also it is showing in the morning that it is only charged to 28% but when i do a reboot it goes up to 90%. Has been happening for the last week. Really annoying.