Shulker Box Magically Disappears: Lose Everything

It was just another wonderful day in my Survival world. I decided to do a hopper test with shulker boxes. I placed a shulker box down:empty. I placed a hopper on top. I stood next to it and jumped, placing a shulker box with valuables. It suddenly vanished, not in the hopper and not in my inventory. What has happened??