Get ACF fields in relationships of returned post

You need to loop for the results and call get_fields() again. So after your code add smt like:

foreach( $locations as $l ) {
   //grab post id
   $pid = $l['ACF_Relationship_Field'][0]->ID;
   // grab fields
   $fields = get_fields($pid);
   // add them back. As ACF_Relationship_Field is an object I will add it next
   // to it, but you can do whatever you prefer
   $l['ACF_Relationship_Field'][] = $fields;

}