Hi @gubbins!
Thanks for the feedback!
When using from: in the searchbox, your own logged in username will also appear in the list so this will add one user to the overall list (as you can also search for messages from yourself).
That said, I'm not able to reproduce the disappearing users as you've detailed in your steps above. I'm thinking it might be because there is a problem with the GitLab omnibus installation.
I'm going to link in @eric, one of our product specialists who might be able to shed some light on the issue.
Let's see whether we can get to the root cause of your issue!