Hi @Taffer, depending on if you use Postgres or Mysql, this doc might help: https://docs.mattermost.com/help/getting-started/searching.html#differences-between-mysql-and-postgres-search.
Not sure if you use Elasticsearch, but this doc might also help: https://docs.mattermost.com/deployment/elasticsearch.html.
Did this issue start to happen after you upgraded to v5.12?