Some users have stated that the Battery Health doesn't seem to support all aftermarket batteries. That and this new feature being a Beta version means there's not much more you an do beyond what you have already tried. I find it odd that it would keep the same value as the original battery, even after a full wipe of the device.
You could try to do a DFU restore which wipes the device and downloads a fresh version of iOS. Make sure to backup your data first and test the Health feature before restoring the device.