The following hooks exist, but appear to be undocumented
acf/fields/user/query
acf/fields/user/query/name={$field_name}
acf/fields/user/query/key={$field_key}
there are 3 parameters, $args, $field & $post_id
https://codex.wordpress.org/Function_Reference/get_users
It works the same way as https://www.advancedcustomfields.com/resources/acf-fields-relationship-query/
