Legion 5 keyboard stopped working

I bought a new Legion 5 a few days ago from a private dealer. I went through the usual process of setting up Windows with an admin and a standard user account. After Windows finished all of its updates I installed Lenovo Vantage to make sure all drivers are current. I updated the bios as well as some other driver. After reboot the built-in keyboard stopped working. Interestingly, fn+screen brightness still works. External keyboard works fine. I have done all the common steps such as Windows self-diagnosis, Lenovo diagnosis, deleting keyboard drivers and even restoring the system to the earliest restore point but no difference. The only thing I have not tried yet is rolling back the bios update. Any other ideas?

Update: I rolled back the bios from FSCN18WW back to the version FSCN15WW it came with but no difference. The first time getting in the Bios, the keyboard worked fine but not in Windows. Ever since the keyboard did not work even in the Bios which tells me it has nothing to do with the OS! Still, in order to be on the safe side, I completely reset Windows and started all over from scratch with the same result. At this point I am certain it has nothing to do with the OS. Maybe it is a hardware defect...

Final update: I called Lenovo support, they did a number of tests and concluded that it must be a hardware defect. Since there was still warranty, a technician came and replaced both the keyboard and the mainboard. Since then no problems anymore.