The filter for the relationship query has some issues. Some things you can do and some things just break it. The problem usually happens when you try to order the fields. Filtering usually works find. The problem happens when ACF attempts to group the posts.
If you really need to order the posts by a specific field I would suggest using a select field and dynamically generating the choices for the field yourself. http://www.advancedcustomfields.com/resources/dynamically-populate-a-select-fields-choices/
