The list of values are fetched with a terms aggregation. This aggregation sets timeout and terminate_after to avoid long running requests. This can result in trimmed results since the aggregation request can terminate before all documents are considered.
In Kibana 7.3.0 and higher, you can configure these settings with kibana.autocompleteTimeout and kibana.autocompleteTerminateAfter. See https://www.elastic.co/guide/en/kibana/current/settings.html for more details