SOLVED: It is not possible to fast-forward or rewind a series; it always jumps to the beginni

[TheDreadPirate]

Unrelated, Handbrake can pass through audio without transcoding to the output file.  You don't have to use MKVToolnix to remux with the original audio.

   

If you're using an Nvidia GPU, I can send you the Handbrake preset I used to use when I was using Handbrake to convert my media to HEVC.

Back to your issue.  It sounds like the container is borked in some way.  If you open a bash shell for the jellyfin container, you can use ffmpeg on the command line to remux and fix any container issues.  No re-encoding.

Code:

/usr/lib/jellyfin-ffmpeg/ffmpeg -fflags +genpts+igndts -i "/path/to/your/video.mkv" -map 0 -codec copy -max_muxing_queue_size 2048 -max_interleave_delta 0 -avoid_negative_ts auto newFile.mkv