Finally, I resolved it like this, on the second query inside the loop
<?php $negatives = get_field( "collections_single_work"); ?>
<?php if( $negatives ): ?>
<?php foreach( $negatives as $post ): ?>
<?php setup_postdata($post); ?>
here goes some coding
<?php $related_posts = get_field('collection_edition_size');
if ($related_posts) {
foreach ($related_posts as $related_post) {
?>
<?php if( get_field('size_of_the_negative', $related_post->ID) == '4x5'): ?>
here goes some coding
<?php
}
} ?>
here goes some coding which is part of the first loop
<?php wp_reset_postdata(); endforeach; ?>
<?php endif; ?>
