Yes sir that is the one I was thinking of, with the MUsixploreForestPine.
Are you using this "MUsixploreForestPine", or are you using a custom music type?
If you're using a custom type, I'd check in on to make sure that the tracks in the type are functioning. Just some google searching showed me that there's a bug in the Creation Kit: the only music files it takes are .wav files.
Another thought, what is the music type of each cell? Looking at the cells in Blackreach I can see those have a Default music type while the Blackreach Worldspace has MUSDungeonIce. Maybe there's a "None" value set for the testing cell's music?
Also your method of testing might be odd. I'd make sure to save outside my worldspace before entering it again to test (make sure it's a clean load every test). Coc markers can help if you don't want to spend time making doors and walking around or anything.