iPhone 12 Pro Max randomly restarts after iOS 26.2 update

hi, i have an iphone 12 pro max and it keeps randomly restarting when i use it. sometimes it stays open for 3 mins to 20 minutes ish then it just turns off again. the battery doesn't drain or anything but it really annoying because it makes my phone almost unusable. as of rn, my maximum charging capacity is 86% and i still have like 70gb of storage left so i feel like it might not be a charging issue. recently, i updated to ios 26.2 around two days ago and i checked and it should be up to date. i havent gotten any wanrings for overheating and i don't think it's overheating. i've been trying to force restart and all that but nothings working. what should i do? :(

(note: the phone is a hand me down so its not new but this never happened before to the previous owner)