Is it transcoding or remuxing? The ffmpeg log would tell us that. I'm assuming if only the container is not supported it is just remuxing. What container is tvheadend streaming? TS? HLS/fMP4? MKV?
Jellyfin 10.10.3 (Docker)
Ubuntu 24.04 LTS
Intel i3 12100
Intel Arc A380
OS drive - SK Hynix P41 1TB
Storage
3x WD Red Pro 6TB CMR in RAIDZ1 (JF Library)