Mi Max 3. Cannot root!

Greetings.

Mi Max 3 came unlocked from vendor.
I have flashed MIUI 8.9.20 using TWRP.
No issues so far. Everything seems to work just fine.

I flashed Magisk 17.2 with TWRP.
Root Checker tells me that 'Root access is not properly installed on this device'

Uninstalled Magisk and reflashed MIUI 8.9.20 to start off clean.

I then flashed SuperSU with TWRP.
Root Checker tells me the same thing!
I've tried a couple of Root Checker apps. All checks are negative.

Uninstalled SuperSU and...

Mi Max 3. Cannot root!