Its been a year since this report about interfaces being bugged and Epic still hasn't fixed it.

General Feedback & Requests
1

https://answers.unrealengine.com/questions/315990/c-interface-cast-returns-null-when-implemented-on.html

This is a major bug with a simple fix. Its a real pain in the *ss for us to maintain our own UE versions for simple bug fixes. Why won’t you integrate such a simple fix?