When a world with placed Shulker Boxes is loaded for the first time in this Snapshot, the Shulker Boxes lose their stored items.

Shulker Boxes keep their contents if they were inside a chest or other storage before the world was loaded in 18w47b for the first time.

EDIT: If it helps, my world was saved in 18w45a, and the Shulker Boxes were fine.