You may also want to double-check with Windows 8.1 again if it will not be too painful, but if the repair office forgot to enable the keyboard backlight for your motherboard replacement, it may need to be sent back again to have this done unfortunately.
If you have the latest atk driver for Win10 though and the connections look good, it sounds like it probably needs to be sent back to have this fixes.
If you talk to support, you may want to confirm the motherboard was replaced before requesting another RMA number.
If you're interested and haven't seen it already, check the thread below for a little more information.
https://rog.asus.com/forum/showthread.php?39519-My-laptop-s-backlight-is-not-working-solved/page2