You need to move the get_field into the loop. You are performing this function outside the loop so the post ID is either wrong or non-existent when you call the function.
// ... code before query & loop
$args = array(
'post_type' => 'evento',
'post_status' => 'publish',
'posts_per_page' => '-1',
'meta_query' => array(
array(
'key' => 'fecha_del_evento',
'compare' => '>=',
'value' => $today,
),
),
'meta_key' => 'fecha_del_evento',
'orderby' => 'meta_value',
'order' => 'ASC',
);
$query = new WP_Query($args);
if ($query->have_posts()) :
while ($query->have_posts()) : $query->the_post();
// put your get_field call here
$artista_evento = get_field('artista_del_evento');
$today = current_time('Ymd');
// the rest of your template code ...
