I found the problem and resolved it.
I was using a plugin which re-order the post-type I tried to call.
This plugin is called Reorder-Post-Type.
It seems to apply a filter on all queries that concerns a post-type affected by the plugin, even in your own code.
If you want to prevent that, there’s a parameter that you can add in your query args, called ignore_custom_sort => true, like this :
$args = array(
'posts_per_page' => -1,
'post_type' => 'post',
'order' => 'DESC',
'ignore_custom_sort' => true,
'meta_key' => 'your_custom_field',
'orderby' => 'meta_value'
);
