I believe its due to the new contributor.role array.  the selection key gets
created and then includes an array ref.  This ref isn't the same ref on
re-entering so the lastSelection just gets lost and the hash grows without limit.

This is after just a bit of browsing:
$VAR1 = {
          'genre,artist,album,track:0:' => 9,
          'artist,album,track:0:contributor.role=ARRAY(0x9c1ef58)' => 4,
          'age,track:0:' => 3,
          'album,track:0:contributor.role=ARRAY(0xa288554)' => 13,
          'album,track:0:contributor.role=ARRAY(0x94cea58)' => 1,
          'artist,album,track:0:contributor.role=ARRAY(0x94ce2d8)' => 5,
          'album,track:0:contributor.role=ARRAY(0x94c9d30)' => 10,
          'album,track:0:contributor.role=ARRAY(0x9aedeb8)' => 13,
          'album,track:0:contributor.role=ARRAY(0x9aede4c)' => 8,
          'album,track:0:contributor.role=ARRAY(0xa01a700)' => 6
        };


I think the right thing here must be to check for the ref and dereference.  it
seems to work, but maybe I'm not getting all of the right data.  patch coming
shortly.