you need to do something like this:
the key is = to loop first through all post, save marker/values into an array (without echo something), output one single map, and output the markers from saved array
<?php
$the_query_map = new WP_Query( array( 'post_type' => 'event', 'posts_per_page' => -1, 'order' => 'ASC', 'orderby' => 'menu_order' ) );
if($the_query_map->have_posts()) :
while($the_query_map->have_posts()):
$the_query_map->the_post();
$the_ID = get_the_ID();
$get_google_map = get_field('event_map', $value);
$output_map[$the_ID]['map'] = '<div class="marker" data-lat="'.$get_google_map['lat'].'" data-lng="'.$get_google_map['lng'].'"></div>';
endwhile; endif;
wp_reset_postdata();
?><div class="acf-map"><?php
foreach( $output_map as $key => $map_marker ):
echo $map_marker['map'];
endforeach;
?>
</div>
you need to adapt my code that it fit your needs, but i hope with that help you can do it at your own.
