Notice that your phone is 6 years old, and not one year old. There’s your hiccup.

Also, I’d rather replace a battery in a 6 year old phone, than put up with it only being at half capacity and being underclocked so it runs like shit.

Further also, you aren’t quite understanding the issue with the phones. Most phones manufacturers leave some “wiggle room” for their power demand and don’t need a full 3.6v for stability (lithium batts are 3.7v nominal). Apple screwed up their power draw and screwed up their battery manufacturing.