Why is my battery health low again if I just got it replaced

If it is not an Apple battery it is not surprising that the Apple Battery health app is not reading it correctly. Depending on the iPhone you have it could be as low as $49 USD assuming Apple will work on it after someone else replaced it. Make a Genius Bar appointment before you go.

Did you have it replaced at an Apple Store? If yes, take it back and show them.

https://www.apple.com/retail/geniusbar/