When a stack of items is on the floor and the adjacent inventories do not have enough room (say 64 stone on ground but only room for 16 in the chest) for the entire stack, the hopperhock will attempt to pick up the stack and will spam particle effects, but will not actually pick it up and will leave it on the ground.

Steps:

1. Have an almost full chest with the last slot partially filled.
2. Place [floating] hopperhock [petite] next to it.
3. Drop a full stack of the same item in the last slot of the chest near the hh.
4. Observe.

Pics: