This worked.
$competition = get_field( 'related_brand' );
$competition_array = $competition[0];
$competition_ID = $competition_array->ID;
$posts = get_posts(array(
'post_type' => 'competitions',
'posts_per_page' => 6,
'meta_query' => array(
'relation' => 'AND',
array(
'key' => 'related_brand',
'value' => '"' . $competition_ID . '"',
'compare' => 'LIKE'
)
)
));