Pixel 3a always on and bootlooping, any suggestions?

Pixel 3a always on and bootlooping, any suggestions?

Hello.

For maybe the 5th time I just took apart my old 3a that my son's been using. I was going to replace the battery, but turned out the battery I'd ordered (OEM and fresh from the wrapper) was bad (puffing up). Needless to say, I didn't replace the battery just now. The old one was managing ok, so definitely wasn't dead. I removed it carefully, so figured I could just reinstall it until I got a better one.

Having put the phone all back together (unfortunately, breaking the cable for the fingerprint sensor along the way, but AFAIK, that would have nothing to do with my problem), I found the phone was bootlooping. Not just that, but once I took it back apart, I found that as soon as I plugged in the battery to the motherboard, it started to boot. Holding any combination of buttons didn't stop the bootlooping. I'm just seeing the start screen (it's rooted) and maybe a flash of the Google logo, then back again. I can't turn it off without unplugging the battery.

So now I don't know what to do with the thing. I thought I'd just put it back together and let it limp along for a while, but suddenly it's hosed. Has anyone else experienced this kind of behavior and found any fix?

Thanks