On Jira Cloud I created a custom field to track who the Designer is on any particular item.
It's user filtering is set to "All active users are allowed".

I assigned this to an item and it shows up there on a test item in the project just fine. However, when I click on it, it is empty. I can't seem to set it to anyone, much less get it to autocomplete. It explicitly states that there are no options. I am set as a site admin and am the only person in the project right now.

It would probably be ideal if it was limited to people in the project but for now I'd settle for being able to set it to any of our users.