Update
I right clicked on "Windows Hello Face Software Device" and selected "Uninstall device", checked "Delete the driver software for this device" then rebooted my laptop once.
Now the device can start but Windows Hello Face still says "We couldn't find a camera compatible with Windows Hello Face" as before.
Before that I have also uninstalled both "Integrated Camera" and "Integrated IR Camera" and re-install with the latest driver (both cameras use the same driver) downloaded from Lenovo website. Didn't help.
Now I'm clueless. No idea where to investigate next... I'm afraid re-installing Windows from scratch is what I need to do.