Query entries from custom post type in an ACF flexible content field

by

You are missing $locations->the_post() in your loop:

if ( $locations -> have_posts() ) {
    while ( $locations -> have_posts() ) {
        $locations->the_post(); // Needed

        echo 'yes'; 
    }
}

One of the things this function does is advance $locations to the next post. Without it $locations->have_posts() will just repeat infinitely for the first post.