The "Stop words", "Word stemming" and "accent removal and character simplification" options can dramatically impact this feature due to how XenForo invokes ElasticSearch
It does this by indexing the title/body fields into a "body.exact"/"title.exact" fields in addition to XenForo's fields, and using the exact text match suffix search option to control if ElasticSearch checks both fields or just the exact text match fields.