Dropping an item into a hopper facing into a shulker box makes the item disappear

I found this bug when attempting to build a super storage system. When you build a setup like in the attached picture and drop an item into the hopper, the item disappears. It isn't in the hopper (comparator output 0, no items in inventory screen) and it isn't in the Shulker Box. Placing an item into the hopper manually, however, works as normal.