Which kernel do you use on Debian? IIRC support for Intel Arc was added in 6.0 or higher. I am using Proxmox (based on Debian) and I had to upgrade from 5.15 to 6.2 kernel to get hardware decoding to work. Have you checked the Jellyfin manual? It’s pretty elaborate on how to get Intel QSV working.
I suppose xrandr can help you here: See the Arch wiki about xrandr