Some people have reported that the battery health settings in the public iOS 11.3 release doesn't load. They also don't seem to work if an aftermarket battery is installed.
If you're really curious, there are other battery health readers, such as the Battery Life app available on the App Store, or coconutBattery if you have a Mac. They don't necessarily produce the same result as Apple's official battery health reading that's in Battery Health Beta or through Apple's iOS diagnostics. I've tried both, and the value seems to fluctuate.
Battery Health - Your Doctor on the App Store
https://www.coconut-flavour.com/coconutbattery/