Why does my phone randomly restart?

A couple of months back I replaced my iPhone 7’s battery because the battery section in settings showed 78% relative to when it was new. So I went to change it and for about a month it worked, except it still died just as fast as it did before. The battery screen continued to show 78% so I just let it be and used it. About a month later I noted that my phone would unexpectedly shutdown and reboot slowly. After it rebooted it would show a battery percentage way off what I had before (I had about 92% battery, then 9% after it rebooted.) It would continue in this loop infinitely until I did something physical to the phone like drop it flat on its back or hit the front. Then it'd kill about 10% and go back to how it was before. I thought it was a software issue but after downgrading from the iOS 13 beta to iOS 12 and watching the same thing happen, I figured there was something wrong with the battery connector or the battery itself. Is it because the battery in my phone isn't designed for an iPhone 7 and maybe for another model?