I’m having the exact issue on 2 separate 8 plus models now... after a 2hour live chat with apple established you can check potential hardware issues by going to setting,privacy, iPhone analytics and improvements then data and if you have any tasks starting with “panic” then it indicates a hardware issue i.e battery etc... The battery i received was as you described the connector was about 10mm too long meaning it didn’t quite line up with the fpc connector and therefore had to be manhandled/slightly bent to connect it (can’t be good for any electric current) I have a new quality battery on order coming tomorrow and will update if it works and resolved the issue as this will support your video.

Tanks do the hey

Do this help

I need your help

This was really helpfull

I have the same problem. When the phone is on it shows 86% then it suddenly drops to 60 something% then the phone restarts and does the same thing, till the battery runs out. i bought a new battery from amazon, and it was working perfectly for 1 day, and it started the same thing, with the new battery. i disconnected the battery and connected the battery back and it started the same thing after a week again. and when it starts going on and off, the phone is not more able to detect the battery when i try checking the health of the battery. i have a feeling that the problem with the phone is the charging AC, i might be wrong. please what do you think. Thanks.