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I also got into this problem 2 times: (including my device was unlocked BL and TWRP installed)
First, i was on MIUI OREO f/w and flashed Xiaomi.EU WEEKLY version - Android 9 firmware (MIUI 10) ROM, i flashed the file named : 'xiaomi.eu_multi_HM6Pro_9.4.25_v10-9.zip' - on restart, my device immediately BRICKED with MI logo shown and error came as - 'System is destroyed! Press power button to shutdown.' - wierd
I tried everything to get the device back using various MI flash tools, but didnt worked.... finally gave it to service centre where they say they replaced the motherboard.... very strange
Secondly, when i started again, till that time now STABLE Xiaomi.EU ROM with Android 9 firmware came, so i thought to flash it again (since it was STABLE and with no bugs of firmwares), i used file named : 'xiaomi.eu_multi_HM6Pro_V10.3.1.0.PDICNXM_v10-9.zip' - on restart, my device immediately BRICKED AGAIN with MI logo shown and this time no error was shown but just MI logo came and it flashes off!!!! - this time also I tried everything to get the device back using various MI flash tools, but didnt worked.... finally AGAIN gave it to service centre where they now flashed - stock stable MIUI OREO based ROM and it is working till now
Finally, I guess now that Xiaomi.EU custom roms are not prepared well enough till now - although the OREO firmware ROM is ok - but definately PIE f/w not recommended!!!!