I have a lot of playlists. I organize them into folders, and subfolders, and sub-sub-folders, alphabetically. e.g. "ABC" --> "A" --> "AC/DC" --> "Back in Black" and so forth. I noticed that on Windows native Spotify app the playlists towards the bottom of my list - "STU" and "WXYZ" - were missing (e.g. "WXYZ" is there, but "W" and all its sub-folders and playlists are missing).
I notice that on Android, all the playlists are there, so its not that I accidentally deleted them. I went to the Web page to restore deleted playlists, and as expected, they are not on the deleted list, because I didn't delete them!
Yet, they are missing from the Windows player. They were gone for a day.
And then, magically, they re-appeared in the Windows player!