Actually, in this case, what you’re looking for is the acf/fields/user/query hook/filter. This hook is undocumented at the moment but it works the same way as acf/fields/relationship/query https://www.advancedcustomfields.com/resources/acf-fields-relationship-query/. The main difference is that the arguments you return should be the arguments that you’d use for the WP function get_users() https://codex.wordpress.org/Function_Reference/get_users
