I have the exact same issue. QC not working.Mi mix 2 quick charge problem on usb3 wire (charging only 300Ma on ampere) , was working before.
When I connect cable that come with phone (usb 2.0 I assume) ampere gives +3000Ma.
Tested with (usb 3.0 blue connector) , I have two of them and they worked before.
Tested 3 different qc 3.0 chargers usb 2.0 stock cable works +3000Ma, usb 3.0 does not work.
Edit. Android versions are wrong at firts post Mi6 and Mi mix2 ,I think. Should be 8.0