Did you change any of the transcoding settings for tone mapping? By default the output of a tone mapped transcode should be pretty close in brightness to a native SDR video.
Jellyfin 10.10.7 (Docker)
Ubuntu 24.04.2 LTS w/HWE
Intel i3 12100
Intel Arc A380
OS drive - SK Hynix P41 1TB
Storage
4x WD Red Pro 6TB CMR in RAIDZ1