On 1.1.1 for 1.18, I found a bug with hopper shulkers when holding them open in inventory with Quick Shulker. It seems like items that get vacuumed up by the hopper shulker are just deleted if the shulker that would be taking them is open in your inventory screen.

Found this out by tossing out items with shift+t so that I could craft them into blocks to condense space but leaving one in the box so it would still pick them up later - I was tired at the time and not thinking that it would just grab them as I picked them back up. But all the items instead just vanish entirely.

Duplicating is easy enough - take any hopper shulker, put a stack of dirt in it, and throw a few pieces of the dirt on the floor, standing so you'll pick it back up automatically. The dirt gets picked up and then disappears entirely.