I was without Magisk since the 1st Android 10 / MI11.
Today solved (MI8) :
1) TWRP (only data) backup
2) TWRP advanced wipe (system, data...only Internal Archive left untouched)
3) TWRP ROM install
4) TWRP restore "data"
after this Magisk is installed and working
5) Installed "MagiskHide...