Currently, the global search doesn't have a modern field-based SearchEngine implementation. In particular, the "Tags" field uses PhabricatorProjectDatasource, not PhabricatorProjectLogicalDatasource, so searching for "Engineering" does not find objects tagged with children of "Engineering".

This isn't completely straightforward to resolve because we need to swap the datasource, resolve the tokens properly, and then translate them to the query correctly (which may be a bit involved).