try reinstall win10 using any amdgpu as video out from beginning.
Just tried it. Plugged in a single GPU and use that as video output, there's no image on my screen if I enable Windows 10 mode in bios. If I disable windows 10 whql support in bios, it works fine but I can't boot into a windows install or windows itself. If I plug the monitor directly into the onboard hdmi, then everything works fine.
I disabled fast boot and it seems to be working!! I get proper video output from one gpu shown on my monitor with 7 gpus hooked up. However now Device Manager only shows 5 GPUs instead of 7.
Bios settings same as before. All Gen 1 with 4G Enabled
Actually, everyone says that GEN must be "2", but i have the same problem as you - Device Manager shows only 5 GPUs no matter what.