User search (autocomplete) for adding users shows no suggestions when typing partial names, but typing the full name works.

If users were imported directly into the ost_user table via SQL (bypassing the UI import), the ost_cdata search index table is not populated. Drop the ost_cdata table and run cron repeatedly until it rebuilds with all user data. Use the built-in import at Agent Panel > Users > User Directory > Import for future imports.