Sort by an ACF field in a tax_query

You have placed orderby in a wrong array.
Please check modified code.

$loop = new WP_Query( array(
    'post_type' => 'partenaires',
    'orderby'  => 'title',
    'order'    => 'ASC',
    'posts_per_page' => -1,
    'paged' => $paged,
   'meta_key'=> 'partners_order', 
   'orderby' => 'meta_value_num', 
   'order' => 'ASC', 
    'tax_query' => array(
        array(
            'taxonomy' => 'type_partenaires',
            'field'    => 'ID',
            'terms'    => $term->term_id

        ),
    ),

) );