For privacy reasons, credit artists belonging to other customers don't appear in the list. You can only find them by typing their artist name in full and exactly, for example in the selection dialog for main, featured, and remix artists or in the "Name" field in the Track Credits. Only the artist name will then be displayed.
You can find plan artists belonging to other customers via the search, starting from two letters, as long as their profile is public and their contract is active.