[Xiaomi Poco F3 / Xiaomi Mi 11X / Redmi K40] jylee1997123:
TLDR below

Some of the backstory of this device:

This is my girlfriend's first android device, tried so hard to convince her to switch from iPhone to Android, I finally managed to convince her, but... this happened...

So since this is not my device and I wanted a pure Android 12 experience (which I bragged about), I installed Pixel Extended Android 12 Beta to the device. (Only android 12 experience that I got during that time)

This is what I did,

Unlocked the bootloader after 7 days ( using Miunlocktool )

With the phone still in factory firmware, I used pixel extended's recovery image which is located here https://sourceforge.net/projects/pex-alioth/files/1-11-21/

Steps that I did ( fastboot flash boot boot.img > fastboot flash vendor_boot vendor_boot.img > fastboot reboot recovery > Format data > adb sideload PEX.zip > Reboot )

After rebooting, the phone is now in PEX4.0 beta and she proceed to use the devices for several months after PixelExperience released their stable ROM for OTA updates and zero maintenance from my side.

When PixelExperience released their stable ROM, took her phone, trying to shut down the phone so I can go into fastboot and I realized that the phone isn't powering on at all which no display, plugged into my PC, no USB indicator as well.

The phone is dead from a shutdown which shocked me. My guess initially is some kind of electric damage that ruined the motherboard. So I sent this to Xiaomi's service center to get this fixed.

After picking up the phone from the service center yesterday, I have been told that the phone is hard bricked and not some kind of faulty hardware. I am pretty sure they reflashed the firmware with Xiaomi flash tool and it is now running on the global ROM 12.5, with an unlocked bootloader

Feeling weird at the hard brick, I thought I messed up somewhere.

On my second attempt to install PixelExperience, I managed to go into fastboot and sideloaded the zip into ADB. This is the command I used.

( enable USB debugging > adb reboot bootloader > fastboot flash boot PE_recovery.img > adb sideload PE.zip > Reboot )

With the sideload showing success message Total xfer: 1.00x I restarted the device and found that the device is now hard bricked once again.

Super confused, I double-checked the recovery img and ROM to make sure that the files aren't corrupted. Prior to installing, I also made sure to update my ADB and fastboot to the latest version.

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TLDR

What could go wrong here?

Upon typing here I think I forgot to do a wipe data before reboot, will this cause the phone to hard brick?

I guess I am sending this phone to the service center tomorrow again, made this thread to know what can I do to make sure this doesn't happen again.

Will I be better off using TWRP instead of their custom recovery image?

Am I messing up something here that caused the phone to hard brick? This image is also the exact image my POCO f3 is currently running which I sideloaded successfully and still running fine.

Should I do an update on MiUI before proceeding to install a PixelExperience?

Should I do a reflash after recovering the phone from Xiaomi's service center?

Could this be some kind of hardware problem?

I ordered my phone online but purchased her phone in the store, are there any differences here?

Any help or input is really appreciated, I have been doing this for quite a while now and never faced such an issue before.