cmus hiding artist songs

I'm using cmus with Debian Buster. In the artists list (view 1), I see a single Al Green, with a single song. If I filter :filter artist="Al Green" I see Al Green repeated twice, first with the previously visible song (from the Pulp Fiction album), and second with his full album. However, when I remove the filter :filter then there's only one Al Green again, with a single song. What's happening here? EDIT I noticed that one Al Green is on A, and the other is on G (on alphabetical order). Elton John appears in both E and J, with different songs in each. Why?