In order for the repeater to be sorted in the admin, you will actually need to update the field so that it is sorted. You can probably do this with an acf/update_value filter https://www.advancedcustomfields.com/resources/acfupdate_value/.
add_filter('acf/update_value/name=my_repeater_field', 'my_sort_repeater_function', 10, 3);
function my_sort_repeater_function($value, $post_id, $field) {
// sort repeater array
return $value;
}