Hi @bram Willemse
The issue is not related to the nested situation of your relationship field. The issue is for some reason, the value is not being loaded from the ‘customer’ pos t object.
Because your code sits within a separate file, it is possible that the $post->ID variable is not correct. You should output this within your loop and double check that the value matches for each customer.
You may need to place this at the top of the file:
global $post
