In a playlist, you select a track and there's a box on the top left corner listing the other playlists where the track is listed. This feature doesn't work anymore in RB6 for the newly created playlists, i.e. the lists that didn't exist in RB5.
Here's how to repeat the issue :
1. In RB6, create a new playlist, eg "Test Playlist"
2. Import new tracks and add them to the Test Playlist
3. Add the track to a playlist that already existed in RB5. eg for me "Club / Progressive / 2020"
If you go to "Club / Progressive / 2020" and select the track, the top left listbox shows the other playlists listing the track : "/", "Test Playlist", "Club / Progressive / 2020"
But if you go to "Test Playlist", the only entry is "Test Playlist".
I used this feature to check if the tracks I was preparing where already stored in the right playlists but it's useless now.