@sun Jellyfin isn't really designed to work with arbitrary videos since it wants metadata, though I'm assuming you're using ytdl-sub which would fill out metadata for it. If it's not finding anything it sounds like something iffy with ffmpeg since I think it tries to parse metadata with ffmpeg from the file before importing it. Or you just gotta recreate the library, I ran into a ghost library bug once.
Just to double check though I dropped a random video file into my movies library and jellyfin picked it up fine so not sure what's up there.