battery draining mega fast (visually visible)

[hash02]

Hello.
I have a lenovo zuk z2 pro phone (yeah i know not the most popular brand/model but moving on)

I have used it on average 2h a day for the past year (phone bought almost 1 year ago)
I'm an enthusiast and like rooting/installing custom roms etc so it ran custom roms like lineageos and aospextended pretty much since day 1.

yesterday i noticed massive power drain and my phone shut down, first time this happened.

today i let it charge (usually done in 40 mins or so, qc3)

i watched some twitch streams for about 30 mins and browsed for another 15 or so. battery was at 15%. noticed since saving mode kicked in. it seemed totally weird since it usually can take 3+hrs pubg sessions on full charge.

i rebooted to twrp recovery while batt was at 3% and it held at 3% for 15 minutes which seems weird since i expected it to drain any moment.

i rebooted to android, charged it for about 5mins till it hit 15% then unplugged and i could see the percentage drop by 1 almost every 2 seconds like a countdown.

i left it charging for an hour and it reached 78% but on the lock screen it says "Charged - you can unplug the charger". it usually said charged at 100%

is this a batt issue ? battery sensor issue ? phone issue ?

This seems weird since i didnt flash anything other than what it was already running for the past month (latest aosp basically it's android 9.0)

[SenKa]

Thats gonna be a battery. Buy one from a trusted supplier of your choice, and look up a guide online on how to fix it. Or, if you're not comfortable with doing so bring it to a shop

[hash02]

full wipe and reflash latest aosp /opengapps and magisk and it fixed it.

on 40% battery, last full charge 2days ago.