iPhone 12 battery at 1% / not charging after replacement, old works

I'm trying to replace the battery on my iphone 12. The original battery works fine but had been used heavily and wasn't lasting as long as it used to.

I installed the replacement battery (from ifixit), but the phone shows up as 1% and there is no "lightning bolt" charge icon when the phone is plugged in. The battery menu in settings shows the phone as charging during periods when I leave it plugged in though. The phone locks and reboots periodically while plugged in, and 0% remaining" is displayed on the lock screen.

I saw other threads online with similar symptoms from different model iphones that indicated that I might have damaged surrounding components or circuitry. I took me a bit to get the new battery connector installed, but I did not see any loose pieces or visible connector damage on the phone.

I decided to test this by disconnecting the new battery and reconnecting the old battery. If the circuitry was damaged, I would expect both batteries to fail, however the old battery reports percentage, charges and the phone seems to operate fine when its installed.

I've tried reconnecting the two different batteries several times, and have tested with the screen on and off to try and rule out shorts as a possible cause.

The ifixit battery shows 3.78V on a multimeter when measured at the connector, while the original battery shows 3.8V (battery is rated at 3.83V). I inspected the connector on the new battery and can't see any obvious damage.

• Any ideas on possible causes?
• Next steps to debug?
• Is it possible that there is an issue with the new battery?