I have the same issue, Unraid 7.0.0 RC2, transcoding not working on Plex offical build, Plex sees my GPU as below.
Nvidia driver 470.256.02 (have tried other drivers too, same result) Ich777
GPU is just chilling.....
Going to try SerpentMage method at the weekend, remove everything, even though its been triple checked in the Plex Docker and re-add unless someone has a better idea ?