I was 5 minutes to early to post the question above as I’ve found the solutions after looking into WordPress Manual.
Seems that if you call another post_type (in my case vacancies) with new WP_Query, WordPress is opening that (lexical) post_type environment. This means that most functions like ACF’s the_field() will try to look inside the custom post type “Vacancies” if the custom field “vacancy-section-2-title” exists. Obvious it’s not so i need to “Reset the Query” back to the original one.
The solution is putting this function wp_reset_query() at the end of the custom posts loop.
<?php
$args = array( 'post_type' => 'Vacancies');
$loop = new WP_Query( $args );
while ( $loop->have_posts() ) : $loop->the_post();
get_template_part( 'template-parts/vacancies', get_post_format() );
endwhile;
wp_reset_query()
?>
Hope it helps other people in the future!
