Import Mapping does not seem to work correctly

Hi,

I am currently using a Magento test store to import products into Mendix over a REST API.  Getting the product (SKU, name, price etc.) works fine. I am struggling to get the associated objects (custom attributes) like categories and description. When I look at the REST response (see image), the custom attributes are there. 

I used the following import mapping (automatically generated based on the attributes i wanted):

When I call the Rest, the association with CustomAttributes remains empty:

Does anyone know what I am doing wrong?

THANKS!