Hello,
When adding a custom attribute to an application profile, that attribute will not show up in the users profile, {{url}}/api/v1/users/{{userid}}.
It does show up is the users application profile for that application (if they are assigned it), {{url}}/api/v1/apps/{{appId}}/users/{{userId}}
So if a user is assigned to app1 and app2, for each of those apps they will have an associated app profile which would contain any custom attributes specific to that application.