When you got some stackable items (like iron ingots or hoppers) in a not-the-first slot of a chest and try to put more of these items into that chest using hopper the item would go into the first slot instead of stacking with the other items.

Steps to reproduce:

1: Place a chest
2: Place a hopper facing into the chest
3: Put a stackable item in the 2nd slot of the chest
4: Throw the same item into the hopper

The item lands in the first slot of the chest

Expected result: The items would stack.

"Works" both on Paper Servers (found it there) and in Java Singleplayer Worlds.

Related: MC-180895