New battery randomly crashing in iPhone 4s

My iPhone 4s battery was, as you might imagine, not holding a charge like it used to. It held a charge for about a day and a half, so usable, but not ideal. I ordered a new battery through iFixit, and this is where my troubles started.

The battery I was using before this new one was the third one I’ve used in this phone, so it’s no stranger to having batteries swapped, and yet for some reason, upon swapping in the new battery I began experiencing random crashes. I tried cleaning the contacts several times, removing and reinstalling the battery, resetting the settings on my phone, but nothing worked. At its best, it managed to run for about a day straight before inevitably crashing again. Other odd issues like the flashlight flickering occasionally also occurred. Worse yet, reinstalling the old battery seems to have made all these issues go away.

What I’m wondering is if anyone has any theories about what this is and what may fix it. I’m worried I may have a defective battery. I’m at my wits end, and I can’t think of anything else I might be doing wrong here.