iPhone XS no power from battery, errors when plugged in

I replaced a screen and battery on an iPhone XS. Confirmed everything worked, gave back to client. Next day, they returned it, saying that it had no power on battery, and when plugged in, boot looped.

After plugging into my computer, I got errors 4015, 4045, and finally managed to update software, disconnect the speaker/proximity sensor/faceID and it worked (so, my screen/battery replacement didn't seem to be the problem, it appeared to be a problem with the faceID, in spite of there being no visible damage). So I replaced the faceID assembly with a new one (I was planning on swapping the faceID IC after confirming it worked), and it worked fine while disassembled. But as soon as I reassembled everything and was about to call the client, it happened again as before.

So I tried software update again, but this time error 4014, 2006, no way to get it to update. So back open, multimeter confirmed 3.7v from the new battery, original battery (also tested 3.66v) made no difference either. Multimeter on battery connector when plugged in confirmed 3.7v so—while I don't have the TriStar tool to check—normally it's not the TriStar IC.

So I'm stumped. No power from battery, boot loop when plugged in but no way to update (error 2006 now consistently).

Any ideas what to diagnose next?