Huge battery drain after unlocking bootloader - Xiaomi Poco F1 Questions & Answers

Hi there,
I purcharsed a Pocophone F1 two months ago and, after unlocking bootloader, I ran into serious battery issues with the phone only not lasting a day when in the same usage it used to last about three days before. So I came back to the shop and they exchanged me with another one after two months (because they first attempted to understand the issue but finally lost the phone).
So, when I got this new phone, like for the previous one, I found the battery life amazing. Then I unlocked the bootloader and flashed Magisk, and it suddenly dropped : I was losing about 1% of battery every 2 minutes.
So I re-flashed the stock ROM, without Magisk or anything, but the problem didn't disappear. I finally wiped everything, flashed the stock ROM, relocked bootloader (which wiped a second time the data), but the problem was still here.
Currently, I'm running the Havoc 2 ROM (Android Pie) flashed with Open Gapps and Magisk, usingSphinx Kernel. It worked great at the beginning of the day, but for no apparent reason my problem appeared again a few hours ago. I attached a few screenshots to illustrate it :
The first one is when going into a terminal and typing `su` then `top` while I was listening to music on Google Play Music (there was no other application opened, no WiFi, no Bluetooth, no GPS, brightness at, let's say, 50%). As you can see I was even in power save mode. I also tried to use other CPU governors like `blu_schedutil`or `powersave` but that didn't change anything (the default and current one is `schedutil` for both big and LITTLE cores).
NOTE: Don't mind about the usage percent and time, I used the "reset" button to check what was going on.
The second one is the result of Kernel Adiutor in the same context, and the last screenshot is when I have only this application opened (no terminal, no Google Play Music). Because of this, the Pocophone heats up, of course.
Also note this problem can happen randomly ; for instance I just re-launched Google Play Music and played some tracks, the big cores remained at near to zero usage percent (while the 8th core was still as its maximum).
To sum up what I tried:
* Changing ROM doesn't help ;
* Using a custom kernel doesn't help ;
* Re-locking the bootloader doesn't help ;
* Wiping everything in TWRP (internal storage, vendor, data, etc.) doesn't help ;
* Going in power save mode doesn't help ;
* Setting the CPU governor on "powersave" or other doesn't help ;
* Closing all other apps doesn't help
Does anyone have an idea of what's going on? The same problem happened with two different phones, which is very strange ; I don't have any background application running (I manually stopped them all, and the problem was here even after flashing the stock ROM alone, with no non-stock application installed).
Thanks in advance for your help

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Try going back to full stock stable with stock recovery via fastboot method. Drain battery to 0 and full charge.
Are you having good cell reception all the time?

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I already tried this and this changed nothing. And yes, I had good cell reception all the time.
Because of some posts I finally found when looking for my apps, I found Google Play Music was eating battery on some random devices (sometimes it can drain battery at an amazing rate on several different devices, but no one knows why). The battery life seems better know. Still, this doesn't explain why I was experiencing the same problem with the stock ROM with stock recovery, without this application or any other.

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The phone is brand new? If not, have you checked the health status of the battery?
I got mine second hand from aguy at work and it seemed OK on stock MIUI, but I noticed much faster drain on Lineage OS although everybody saying is one of the best for battery life, finally has seemed to get better.
I am using Havoc with pico GApps, D8G kernel (no oc) and FDE.AI(paid). My idle times are much better, and overall use too, the performance in games is much better too, but I've opted for some over clocking to boost gaming so the drain on heavier loads is greater as expected.
Enviado de meu POCO F1 usando o Tapatalk