Nobody has the problem on MI9?
You just have to set the flashlight to a long press, switch off the screen and wait 10 seconds, then try to switch on the flashlight with the AI button.
I did it as you mentioned -> switch off the screen and wait 10 seconds -> voilla -> light is on
ah wait... but if I take my phone, the display switched to on... now I switched this feature off, and nothing happens - no light as on your phone