I also faced the same issue. The backlight used to remain on which caused some battery drain, I couldn't increase/decrease the brightness and also auto-brightness sensor didn't work.
To solve this, I clean flashed without locking bootloader to official Nougat MIUI Global Beta ROM and then after flashing custom recovery; wipe system, cache, dalvik cache and data via recovery if you don't want to delete internal storage files and lastly flash the ROM . If you want to save your internal storage files, just select save userdata instead of clean all in Mi Flash Tool.
If you still have the problem, repeat the same but first flash Marshmallow ROM, then the Nougat ROM and lastly the custom ROM. I think this issue arises due to some kernel incompatibilities with the custom ROM.