WRONG! You can just do a dirty flash or OTA. No bootloop and everything should work fine.
After the reboot, just re-flash TWRP 3.2.3 that's working with Android Pie by using fastboot command and TWRP comes back.
MI 8 SE dirty install without any problem!
Android 9 is super smooth!!! I mean it really smooth!!!
Bluetooth issue when turn on airplane mode also has been fixed! THANK YOU SO MUCH!
I have same BT issue on my Mi 8 SE. Tried wiping cache/dalvik in recovery but BT still could not turn on once I actived Airplane mode... I think it's a bug of the rom