Admin Panel – Disable Moving Selected Terms To Top of Metabox

Try adding this in your functions.php file:

// Let's stop WordPress re-ordering my categories/taxonomies when I select them    
function stop_reordering_my_categories($args) {
    $args['checked_ontop'] = false;
    return $args;
}

// Let's initiate it by hooking into the Terms Checklist arguments with our function above
add_filter('wp_terms_checklist_args','stop_reordering_my_categories');