Hello docdosco,
This is strange from what you have described. I am assuming you have the OSX version on your Mac?
I assume you have cleared the search box at the top (although the number badge on the right of the playlist should still show the number of songs in there) for the playlist?
It is possible you have removed the songs from the 1300 jazz playlist but they should exist in the song index. Do you see them listing in the song index? This contains all songs whether in playlists or not.

Yes you can download the forum 1300 Jazz playlist again. It will list with the other playlists you have. If you have made changes to the songs previously, they will be retained with the original title name, but the fresh ones coming in with that same title will have a number appended (usually 1) to its title, so the imported playlist will have some titles with 1 after them. You will then be able to see them listing underneath each other in the Songs index. Any duplicate songs, exact duplicate songs, will not be imported, but will still list in the fresh jazz playlist you have imported, so it will remain intact as 1300 songs. So you might not see the Songs total increment by much, only by the number of songs you had edited.

But I would make a backup for yourself before doing anything just in case something odd has gone wrong somehow and you have to revert to what you have.