Thanks @lindy65 (but note we're not using the GitLab omnibus installation)
In other news, I reverse-engineered the MySQL query that I believe will get run on the server for such autocomplete queries, having traced it back to https://github.com/mattermost/platform/blob/master/store/sql_user_store.go. For our Town Square channel, with just "c" as the search term, I believe it would be this:
SELECT Users.FirstName FROM Users, ChannelMembers WHERE ChannelMembers.ChannelId = '8r95qhjqgirhmpx6nxt6f83kuh' AND ChannelMembers.UserId = Users.Id AND MATCH(Username, FirstName, LastName, Nickname, Email) AGAINST ('+c*' IN BOOLEAN MODE);Interestingly, the results of this again include only 4 users with usernames starting with c (not all 18). If I understand the query right, it should include all 18 users whose usernames start with c. I played with some variations of the query and got some rather odd results. I wonder if there's some issue with our MySQL version, or the index is corrupt or something?