Hi Pat,
There is a logic error in the details you mentioned above:
Ateliers is parent of Plannings and pl-atelier is a custom field of Plannings
Are we talking about this case:
Ateliers is "child" post type of Plannings,
and you are querying child post type "Ateliers" and display the custom fields "pl-atelier" of parent post type of "Plannings"
If it is, I have tested in my localhost, it works fine when I set custom fields "pl-atelier" as a text field, please try describe detail steps to duplicate same problem in a fresh wordpress installation, thanks