Shulker boxes, previously filled and renamed, can be randomly reset for unknown reasons. The items all become lost and the shulker box gets its name reset back to default. So far, instances have included travelling through the Nether and placing the shulker box in an Ender Chest, although normally these actions have not reset the Shulker Box. So far I have only experienced this bug in multiplayer, not single player. It has happened to at least one other person on our server.