NVIDIA - Ubuntu 20.04 - HDMI second monitor with blank screen, but xrandr detects it [Bug reported on Launchpad and freedesktop's gitlab]

In the file /usr/share/X11/xorg.conf.d/10-nvidia.conf, add the line:

Option "PrimaryGPU" "Yes"

In the file /usr/share/X11/xorg.conf.d/10-amdgpu.conf, change the line

from

Driver "amdgpu"

to

Driver "modesetting"`