iPhone 8 Plus battery replacement problem

If you changed the battery yourself, you likely got a garbage battery.

Apple does not sell iPhone parts at all. Cheap knockoffs abound on the internet...

Unfortunately, if that's what you did, Apple won't touch that phone.

There are occasionally salvage parts, but who really wants a used battery?

Apple doesn’t supply batteries to third parties. At Apple Stores or AASPs the only warranty that would apply is the standard one from Apple, which is 90 days unless it was replaced under warranty which could be the remainder of the original warranty unless 90 days is more.

If it’s an aftermarket battery, many don’t work properly with Apple’s battery health mechanism.

What store?

Only Apple and Apple authorized service providers have access to genuine Apple parts.

Take it back to whoever replaced the battery.

None of the aftermarket batteries will work with the Apple battery app.

You can try, but it’s my understanding that third party battery installation is the least likely “unauthorized service” where Apple might still perform service.

Normally they will not accept it but we aren’t Apple employees. Make an appointment and ask.

The only thing you have to lose by asking is the time you will have to invest to take it in to the Genius Bar.

Odds are, they will politely turn you away.