Battery health. Drop

iPhone X was released in 2018. Battery health typically declines little by little after each full battery charge. What's unusual is not that your battery is now in need of being replaced, what is unusual is the reading should have been declining over time. None-the-less, it would be reasonable to need to replace the battery in a 4 year old iPhone at this point.

Where did you buy the phone?

I haven't seen it before, but there's the possibility of a battery's onboard memory being "re-flashed" to reset the battery health reading such that it seems like it's new. It wouldn't represent the true capacity, but would likely just update really quickly when the power management system figure out that it's not accurate.

A 100% reading followed by massive reduction on an old phone suggests a "seller refurbished" phone with non genuine battery.

Take it back