Unable to mention a team mate (or find them as a reviewer) on a public repository.

I've a team mate I added to a review, but cannot mention any more in a public repository.

When typing the user's full name only 5 results are shown, and this user is not one of them. (there's definitely no users below the fold, as using scroll wheel or keyboard nav only shows 5 people).

I was able to add the user to the review as they were a suggested reviewers.