Right now, Lexeme namespace - e.g. on test.wikidata.org - is indexed in "general" index, together with non-item namespaces. We may want to index it in "content" index, together with items & properties.
If we decide to do it, namespace for Lexemes should be added to $wgContentNamespaces or $wgNamespacesToBeSearchedDefault for wikidata. This is recommended to do before reindexing, otherwise new reindex would be required to move the data to different index.