I'm out of ideas. What happens when you play HEVC with Edge or Firefox? I'm assuming it direct plays with Edge and transcodes with Firefox? Stick with JMP?
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)