iPhone 15 Plus randomly shuts down after full charge

I had from the beginning of 2024 February and also guys my back cover from my 15 plus was coming off but I’m not exactly sure how my battery started to act up never got no warning or no messages about any water leak or anything like that I always had my phone in my ottter box defender pro series so it will need to plugged in a charging port in order to stay on but the battery percentage does go up regularly and it it show’s that I’m 100% charged and ready to go but right when I take the 15 plus off the charging port or (cable ) I will give it like 30 seconds to a minute until it just turns off by itself my battery health is like is at good minimum 86% but when my phone (15 plus) ‘ does turn off by itself at times it will decide to turn back on if it’s possible for it to when it does turn back on my battery percentage would be like 10% to 13 % percentage even after I just had charged it full