ACF meta_key and meta_value break loop

you need to add a LIKE in your meta_query

$args = array(
    'post_type'=> 'recipe',
    'meta_query' => array(
                     array(
                        'key'     => 'special_diets',
                        'value'   => 'vegan',
                        'compare' => 'LIKE',
                    )
                ),
);
$query = new WP_Query($args);

as explained here Count custom post types with a specific meta value