Q: OS X Caching Service not working for foreign country app store downloads

We are running OS X 10.11.6 with OS X Server 5.1.7 and are based in South Africa

The caching server works perfectly fine if I log into iTunes / iOS App store with my South African store account.

When logging into iTunes with my valid US account, app-downloads are not cached. For the test I used apps which are globally available (Twitter, Facebook) and would expect that those apps would be cached irrespective of the origin of the logged in iOS / OS X account.

Can someone confirm how this is supposed to work as we have legitimate foreign users working in our offices / visiting us and we would like to cache this content as well.