I am also on Mi 6, but no longer use MIUI. Last time I used MIUI 12 beta battery performance is quite good compared to MIUI 11 stable.
Lucky you had 50% discount, new Mi 8 still being sold at high price in my country. I prefer Mi 9T as replacement.
Problem solved by install mi doc viewer. Now I have another problem, I want to know why Get Apps (i installed it) disppear from Home screen. How to bring it back? It is not deleted, I still can see the app in "manage apps".
Thanks!