I have a sidebar widget I've whipped up that uses ACF's Post Object to get some specific content from my site.

However, I've noticed when I'm not logged in that it returns empty. Why is that? I can't see any controls that would cause that.

There are no logged_in conditionals in this part of the theme.

while(have_rows('promo_widget', 'options')): the_row();
    $promos = get_sub_field('posts');
endwhile;

$promos is empty when not logged in.