It's not a big issue - I've just got asked about it a few times already.
We've added a new custom field to our project that's of a User Picker type. Unlike the standard user pickers (like the Assignee field) it is not auto-populated after clicking it. User needs to type at least 1 character and only after that the list of users appear on the screen.
What's worse - it shows "No options" message so the users often don't even try to start typing.
I've already checked - users do have "Browse users and groups" permission.
Any ideas how to fix this or is a problem on Atlassian side?