For large boxsets the MB web service does, for performance reasons, not return the per track relationships.
Picard should show a warning in such cases. It should be checked what conditions the server applies, and in such cases and if there are indeed no relationships in the server response the warning should be shown.
Maybe combined with offering the user the option to reload data track by track, see the comment on PICARD-657
See also discussion at https://community.metabrainz.org/t/composer-and-other-values-not-retrieved/523661/