If you are coming from official rom:
- Unlock bootloader (official way)
- Flash TWRP recovery
- Boot straight to TWRP recovery without going to MIUI (system) else MIUI will replace your recovery with Mi Recovery
- In TWRP recovery select Wipe - > Format data - > Answer 'Yes' - > Reboot your...
Global Mi 9 6/128 here also.
I dirty flashed with updater app (wipe cache/dalvik).
And clean flash isn't wiping system, never wipe it as it is mentioned in original post [emoji6]
Just swipe to factory reset to have clean install.
Or if you are coming from official rom, then format data...