The only thing stored for each post is the id of the related item. There isn’t any way to sort this by anything else unless you build a way to do it.
For example, create an acf/save_post action that gets the user and saves another custom field value, not an ACF field, using update_postmeta() with a new field name to hold the value you want to sort posts by. Then you use this field in your query to sort the posts.
