Currently USTRUCTS do not support saving expansions properly. While we won't implement a separate pathway for USTRUCTS to save their expansions, a general solution to save expansions for class or structs would not only address this, but some of our other expansion and map\array related bugs that need addressing in the near future.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-197300 in the post.