I have the same problem , and the cause is 2 other input devices i have connected to the computer MY Joystick and some thrustmaster usb MFDs, if i unplug theses then my keyboard work in bios ,with these connected keyboard first comes on in windows ,i have written to Asus and got three answers on things to try i treid them but the problem persists.
i have not heard from asus and it looks like they have closed the case with out finding the cause .
i think it is in the bios the problem lays.
start of with just your keyboard and mouse and add your other stuff one by one restarting everytime you put another device on untill you find which one it is.
Did you RMA the board and did you replace it with another Crosshair V
ROG MAXIMUS XI FORMULA
CPU: Intel 9900K @4.7 ghz
2 x Corsair Vengeance RGB PRO DDR4-3200
C16 BK DC - 16GB
1 x Aurous 2080Super 8GB
1x 500GB Samsung 980 M.2 NVMe
1x 500GB AData 8200pro M.2NVMe
2 X 500 GB SSD
1000w PSU from corsair
Tower Lian Li PC-O11DW Dynamic
1 x dell 34" curved + 1 x 24"
CPU and Motherboard.
NZXT Kraken X73 RG