Bootloop after sitting untouched for 1 week - iPhone XR

I replaced the back glass on my XR (which requires you to remove the logic board, battery, screen, etc and then put it all back together once the new glass is on). I did just that, and when the phone was fully reassembled, I powered it up and everything worked PERFECT, including Face ID. So I powered the phone off and took the battery back out, because I had dented it slightly during the repair and I wanted to buy a new one just to be safe.

However, when the new battery arrived in the mail, I installed it and the phone simply DID NOT POWER ON. I thought maybe it was a dead battery, so I plugged it in but then the phone started bootlooping! It continued bootlooping the same way when I disconnected the battery. Even worse, I put the old battery back in and the phone STILL did not power on (the old battery was almost fully charged, when I tested with it a week ago it said 95%)!

Clearly something must be interrupting the connection between the battery and the motherboard, but there is no visible dirt or dust in the battery connection, so I don’t know what to do! The phone just thinks there is still no battery connected! I literally only let it sit for 1 week with the screen attached (so no dust could get inside) but no battery. Does letting your phone sit for a week without a battery cause a bootloop? What could possibly explain this?