tiki-admin.php?page=search -> General Settings -> Don't index non searchable fields

Indexing will skip adding all tracker fields that are not marked as "searchable". This will free index space but also make it impossible to use those fields in search index queries.
Preference name: unified_exclude_nonsearchable_fields

Then, go to the list of fields in each tracker, and toggle the value for the "Search" column.

Rebuild Unified Index