OK, I finally got jellyfin to see the backed up playlist. For some reason when I try to add a song to the playlist though, it just puts up the window for adding a new playlist. Like it doesn't see it, even though playlists is shown on the main page, and the playlist works playing music that is already on the playlist.
Can you walk me through EXACTLY how you backed up and restore your playlist? I just did a test and I was able to restore a playlist without issue.
Here is what I did for you to compare.
1) Create new playlist (to ensure playlist ownership is tracked)
2) Stopped Jellyfin
3) Copied "OwnerUserId" line
4) Restored playlist.xml from backup
5) Ensured "OwnerUserId" line matched
6) Started Jellyfin and Scanned all libraries
Jellyfin 10.10.6 (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