<?php // I intentionally didn't re-indent your code

$post_objects = get_sub_field('choose_blocks');
$string = array(); // array of post IDs
if( $post_objects ): ?>

    <?php foreach( $post_objects as $post_object):

        $post_id = $post_object->ID;

        $string[] = $post_id;


    endforeach; ?>

    <?php wp_reset_postdata(); // IMPORTANT - reset the $post object so the rest of the page works correctly ?>
<?php endif; ?>

 <?php
     $args = array(
        'post_type'=>'presentations',
        'post_status'=>'publish',
        'post__in' => $string
    );
?>
 <?php // the query
    $all_presentations = new WP_Query($args); ?>

    <?php if ( $all_presentations->have_posts() ) : ?>

        <?php while ( $all_presentations->have_posts() ) : $all_presentations->the_post(); ?>

            <?php get_template_part( 'includes/content', get_post_format() ); ?>

        <?php endwhile; ?>
        <!-- end of the loop -->


        <?php wp_reset_postdata(); ?>

    <?php else : ?>
       <!-- do nothing -->
    <?php endif; ?>

<?php endif; ?>