Elasticsearch since Phabricator uses that as backend:

phabricator_cluster_search:
  - type: 'elasticsearch'
    path: '/phabricator'
    port: 9243
    version: 5
    hosts:
      - protocol: 'https'
        host: 'search.svc.eqiad.wmnet'
        roles:
          read: true
          write: true

We had a task to verify compatibility with ElasticSearch 7.10 then T303445#7807237 states Phabricator uses MySQL. From https://phabricator.wikimedia.org/config/cluster/search/ :

@valerio.bozzolan: Did you intentionally set yourself as task assignee?

Decreasing the scope of that query to https://phabricator.wikimedia.org/search/query/aYltXfrCu2Iv/#R, I get four tasks, missing T304193.

The fulltext search code executed first runs
SELECT ngram FROM maniphest_task_fngrams_common WHERE ngram IN (' er', 'err', 'or ', 'ror', 'rro');
(note the common suffix) which returns three results: err, or, rro.

It seems these results get excluded from the ftngrams of the actual followup search, as the following query only includes the remaining two ngrams not found in the previous step.
(And if I manually replace those two ngrams with the three "common" ngrams I get zero results.)

SELECT task.*, IF(ft_rank.termCorpus LIKE '% error %', 2, 0) + IF(ft_rank.normalCorpus LIKE '% error %', 1, 0) + 0 AS
_ft_rank, ft_doc.epochCreated AS _ft_epochCreated, ft_doc.epochModified AS _ft_epochModified, ft_rank.rawCorpus AS
rawCorpus
FROM maniphest_task task  
JOIN edge edgelogic_ancestor_41 ON task.phid = edgelogic_ancestor_41.src AND edgelogic_ancestor_41.type = 41 AND edgelogic_ancestor_41.dst IN ('PHID-PROJ-gzlidnmqz3ztdvx2nubo')
JOIN maniphest_task_fdocument ft_doc ON ft_doc.objectPHID = task.phid 
JOIN maniphest_task_fngrams ftngram_1 ON ftngram_1.documentID = ft_doc.id AND ftngram_1.ngram = ' er'
JOIN maniphest_task_fngrams ftngram_2 ON ftngram_2.documentID = ft_doc.id AND ftngram_2.ngram = 'ror'
JOIN maniphest_task_ffield ftfield_1 ON ft_doc.id = ftfield_1.documentID AND ftfield_1.fieldKey = 'titl'
LEFT JOIN maniphest_task_ffield ft_rank ON ft_doc.id = ft_rank.documentID AND ft_rank.fieldKey = 'titl'
WHERE (((ftfield_1.termCorpus LIKE '% error %')) OR ((ftfield_1.normalCorpus LIKE '% error %')))
GROUP BY task.phid
ORDER BY _ft_rank DESC, _ft_epochModified DESC, task.id DESC
LIMIT 101;

Rewriting that query to
SELECT task.id AS taskId, ft_doc.*, ftngram.* FROM maniphest_task_fngrams ftngram INNER JOIN maniphest_task_fdocument ft_doc ON ftngram.documentID = ft_doc.id INNER JOIN maniphest_task task ON ft_doc.objectPHID = task.phid INNER JOIN edge e ON task.phid = e.src AND e.type = 41 AND e.dst = "PHID-PROJ-gzlidnmqz3ztdvx2nubo" INNER JOIN maniphest_task_ffield ftfield ON ft_doc.id = ftfield.documentID AND ftfield.fieldKey = 'titl' WHERE (ftngram.ngram = " er" OR ftngram.ngram = "ror") AND ((ftfield.termCorpus LIKE '% error %') OR (ftfield.normalCorpus LIKE '% error %')) ORDER BY task.id;
which returns four distinct task results.
T304193 is not among them.