Thanks a lot James !
I can order now the outpout on the front-end 🙂
I update my code like this :
$the_problems = get_field('related_taxo');
$order = array();
foreach( $the_problems as $i => $problem ) {
$order[ $i ] = $problem->count;
}
array_multisort( $order, SORT_ASC, $the_problems )
thanks