Still audio transcoding. And because the resolution is lower, the GPU is transcoding the video faster which requires the CPU to keep up with the audio transcode.
Jellyfin 10.10.5 (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)