I’m also has this problem - black screen on HDMI output.
I have Ryzen 5 with Vega 8 and NVidia 1650 Max-Q (with HDMI) on my laptop, and after installing driver “video-hybrid-amd-nvidia-450xx-prime”, I have features to work with my dgpu. But, when I plug in HDMI display, there is just black screen. It’s detecting by “xrandr” as “HDMI-1-0” (with correct info about display), but also “HDMI-1” detecting as “disconnected”. In GNOME System Settings in Display menu it also available. And it changes correct when I disconnect HDMI display, and reconnect again. It works when I add line
Option "PrimaryGPU" "Yes"
in “10-amdgpu-nvidia-drm-outputclass.conf” at “nvidia” line, but after rebooting login screen appears at HDMI, and all apps running using NVidia card, there is no more AMD. Any ideas, how to activate NVidia card at HDMI port, but use default graphics as AMD?