Picard 2.2: If a folder “[non-album tracks]” is available at the third pane, it can only be removed, if it’s empty. At releases prior of 2.2 such a folder could be removed if it’s not empty.

https://community.metabrainz.org/t/picard-2-2-non-album-tracks-can-only-removed-if-its-empty/444009