Why did my new battery fail so quickly?

In this case the cells chemistry broke down prematurely.

Yes, due to pandemia, the laptop has been working on my desktop with AC power the most part of these two years. That might explain the premature failure and its weird behavior: After about half an hour of working off the battery, it will suddenly fall from about 80% to 7% battery charge within a minute, and then resume the normal discharging rate until depleted for about another half hour.

I came to this thread via searching for the serial number on my newly installed battery on my Early 2015 MBP, model A1502. It's the same serial number, although quite different model from yours.

My original battery was used in the same way as you did this one. On my desk, in the office, plugged in almost 365 days a year. After 4 years of office use I bought the laptop and continued using it for personal use. The original battery degrader to almost 65% original capacity. It was all swollen.

It's very hard to get after market batteries with original quality, but then again, the price of an original battery would have set me back the market price of the laptop as it is, so it makes no sense. I'm curious how well the new battery will hold out. So far, so good.

Yes, running a system strictly on AC can reduce its lifespan, but a bad charger is often the cause of a blistered battery cells. What happens is the knockoff charger does not properly charge the battery not only damaging the battery but the logic board as well. Heres the dope: What does knockoff charger do to Macbook Pro Retina?

Dude, I only used the original charger. Still do. All lithium batteries swell in time.