A friend of mine recently replaced the battery in my iPhone 6 Plus. Battery Health in Settings showed information on the battery pre-replacement but not after, even after a full charge-discharge cycle and reboot. It seems that this feature just will not get any information on third party replacement batteries.
But you can still get information on your battery. Aida64, free in the App Store, will show lots of detailed system information, including battery charge and wear level. My old battery showed about a 50% wear level (meaning it only held 50% of max charge) but the new one now shows a 0.51% wear level (meaning it holds 99.5% of max charge). I'm sure there are other apps out there that will show similar information, but Aida64 is basically my go-to on both iOS and Android.