This has happened to me twice so far and it has essentially ruined my world both times. On rare occasions when a shulker box is placed down, the box will not open under any circumstance and when the box is destroyed with the appropriate tool, it completely disappears, getting rid of all of its contents. Obviously, this is extremely gamebreaking and needs to be addressed because many people like me keep their most valuable items in shulker boxes for ender chest storage.