I am not sure if this also applies to jellyfin, but I had the exact same issue with the Plex App.
The problem is that the TrueNAS middleware fails to detect the UUID of the nvidia GPU, and so it does not get passed to the container.
The ‘fix’ is to setup Plex (and mostlikly this applies to jellyfin as well) inside portainer, and then pass the UUID to the container.
See this for how to get the UUID: TrueNAS 24.10-RC.2 is Now Available! - #92 by Chris_Holzer