Jellyfin 12.1 landed on 15 September, six days after 12.0. The release is 47 fixes and no new features. The notes still say to take a full backup before upgrading.
What it fixes
Several of the fixes are for what 12.0 rebuilt. Database optimization uses less memory now, and the pre-migration backup’s integrity is fixed. That backup is what you have left if the migration goes wrong. Invalid data is cleaned up before migrations run instead of during. Creating a collection works when the collections library was just added, and playlist encoding is read correctly. Versions of a video stop listing separately from their group, and manual merges are kept.
The rest is ordinary patch work. Forced-only subtitles now override a remembered subtitle choice, transcode throttling turns on for systems that support it, library items survive a failed directory read, and metadata provider order applies without a restart. Revoking a device’s access now ends that device’s existing sessions. Live TV’s EPG gets fixes too.
Who should move
If you’re on 12.0, take the patch: the migration and the backup both get fixes. If you’re still on the 10 series, nothing here changes the upgrade itself, and the same backup warning applies. Install 12.1 when you move.