I think the documentation needs to be updated for how to query the relationship field https://www.advancedcustomfields.com/resources/relationship/.
I’m on WordPress v4.8 and ACF v5.5.14.
If I run
<?php
$posts = get_field('relationship_field_name');
if( $posts ): ?>
and there are no posts then WordPress shows Notice: Uninitialized string offset: 0 in /Applications/MAMP/htdocs/nyss/wp-includes/class-wp-query.php on line 3162.
I fixed this by calling get_field directly in the if statement.
<?php
if( get_field('course_faculty') ):
$posts = get_field('course_faculty');
?>