Can you upload your ffmpeg log to sourcebin and link it back here? And a screenshot of your transcode settings in Jellyfin and the media info the one of the videos.
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)