I have the same motherboard with 13 (NVIDIA) GPUs running without issues with Win10. If only 12 seen in device manager, put the fans spinning with afterburner. Check all the cables for the one which is not spinning. Pretty sure it's faulty riser or usb cable.
win 10 pro 1709 + latest amd driver
I am suffering with same kind of issue, I have 11 amd gpus on h110 pro btc+ after reboot sometimes it shows 8/9/10 but never all 11 gpus under hiddent items I can see missing gpus which did not come back even after uninstall & reboot, randomly gpus stop working on crash/reboot.
Did you put something between your riser cables, I am suspecting riser are touching each others and made windows to crash/reboot.