Currently, on the global stats page (https://listenbrainz.org/statistics) the top albums have cover art but the top tracks do not.

This is unexpected. Inspecting an example listen, we can see the MBID mapping does find a match, but the Cover Art Archive data in it is `null`.

{
  "listened_at": 0,
  "track_metadata": {
    "artist_name": "Nirvana",
    "track_name": "Smells Like Teen Spirit",
    "release_name": "Nevermind",
    "release_mbid": "eccae410-7577-4daa-b602-92d305828331",
    "recording_mbid": "5fb524f1-8cc8-4c04-a921-e34c0a911ea7",
    "mbid_mapping": {
      "caa_id": null,
      "caa_release_mbid": null,
      "recording_mbid": "5fb524f1-8cc8-4c04-a921-e34c0a911ea7",
      "release_mbid": "eccae410-7577-4daa-b602-92d305828331",
      "artist_mbids": [
        "5b11f4ce-a62d-471e-81fc-a69a8278c7da"
      ]
          }
      }
}