VFAT? You need to switch from VFAT to F2FS Partition (All Partitions - and mainly DATA) to use Miui Beta and Stable Roms.
Just note you need to use EXT4 Partition for Custom Rom.. and when you use Miui 9.12.4 or any Miui Rom, switch back to F2FS Parition before Rom installation.
You can use Magisk 20.1 after... just make sure that System, Vendor and Data is mounted before flashing. Just make sure that you do NOT start erasing apps via root, or you will get a soft brick and you have to install the rom again... do not install Viper4android, or your device wil get soft bricked again.