post_object.php line 170
5.2.1:
$groups = acf_get_grouped_posts( $args );
5.1.8:
$groups = acf_get_posts( $args );
5.2.1:
get_posts( $value ) is removed
5.1.8:
render_field()/format_value() uses $this->get_posts
5.2.1:
render_field()/format_value() uses acf_get_posts
api-helpers.php
Not sure if it makes a diff, but it looks like in your 5.2.1|acf_get_posts() you loop the product of $posts=get_posts, while in 5.1.8|$this->get_posts() you loop the $value and don’t deal with the product of get_posts.
