To be honest, I don’t think it’s possible to sort posts by a user field unless you want to sort them by user ID. My suggestion would be to create an acf/save_post filter https://www.advancedcustomfields.com/resources/acfsave_post/. In this filter get the user from the ACF field and create a value in a separate custom field using a standard WP update_post_meta() call an add the value you want to sort by to this other field. Then you’d be able to use this field in your query for ordering the posts.
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