There’s a bug (as of 2015) in SharePoint that causes a percentage of users to not display in search results when searching by department/title or displaying a list of all users with the search part (*). In my case, I was trying to display staff in a specific department on a search results page, but was missing people (not always the same people, either). I had a support ticket in for it and was told that SharePoint was interpreting some of the users as duplicate results and removing them from search.
How to fix it
Microsoft support gave me a pretty easy fix for this bug – “grouping” the search results by their document signature (yeah, apparently this works for people, not just documents). This is done in the Refiners tab of the query editor in the Search Results web part. I was told to set the “show these results” to 10, and it works, though I’m not sure why 10… I am guessing that each profile has a unique document signature, so 1 should work too. Shrug.
A note on Premier Support
Premier Support has been pretty great for me, I’ve learned quite a bit from having things break and getting them to help fix it :). By and large even the tier 1 people have been pretty great… there’s been a few exceptions of course, but the pool of people they pull from for tickets seems pretty small. I pretty reliably end up getting someone I’ve had a case with previously, which is nice. I’ve had some funny conversations at SharePoint meetups too, where we talk about who the best premier support reps are (we all agree Eric is great).
Some other gems I’ve learned from support tickets are to never start a list name with a number or underscore (thanks Justin!), that the Chrome Office plugin sucks (users tend to have ever-changing issues opening docs from Chrome in the Office application), and that you can only delete a Delve board if everyone unfollows it (thanks Thane!).